feat(core/presentation): Migrate form validation API to class-based API

This is an iteration on the form validation API.  The primary changes are:
- Migrate from `buildValidators(values)` to `new FormValidator(values)`
- Make fields optional by default
- Migrate from `.required([validator1, validator2], 'field is required')`
  to `.required('field is required').withValidators(validator1, validator2)`
- Add `spelAware()` to ValidatableField which allows validation to pass
  when the value contains a SpEL expression, even if other validators
  would fail the validation.
- Add a magic string that can be used to short circuit validation of a field
  - Used to implement optional() and spelAware()

app/scripts/modules/core/src/notification/modal/EditNotificationModal.tsx

@@ -2,7 +2,7 @@ import * as React from 'react';
 
 import { Formik, Form } from 'formik';
 import { Modal } from 'react-bootstrap';
-import { buildValidators, IModalComponentProps, ReactModal, SpinFormik } from 'core/presentation';
+import { FormValidator, IModalComponentProps, ReactModal, SpinFormik } from 'core/presentation';
 import { INotification } from 'core/domain';
 import { SubmitButton, ModalClose } from 'core/modal';
 
@@ -27,11 +27,12 @@ export class EditNotificationModal extends React.Component<IEditNotificationModa
   }
 
   private validate = (values: INotification): any => {
-    const validation = buildValidators(values);
-    validation
+    const formValidator = new FormValidator(values);
+    formValidator
       .field('when', 'Notify when')
-      .required([(value: any[]) => !value.length && 'Please select when the notification should execute']);
-    return validation.result();
+      .required()
+      .withValidators((value: any[]) => !value.length && 'Please select when the notification should execute');
+    return formValidator.validateForm();
   };
 
   public render(): React.ReactElement<EditNotificationModal> {

app/scripts/modules/core/src/pipeline/config/stages/googleCloudBuild/googleCloudBuildValidators.ts

@@ -1,7 +1,7 @@
-import { buildValidators, IContextualValidator, IStage } from '@spinnaker/core';
+import { FormValidator, IContextualValidator, IStage } from '@spinnaker/core';
 
 export const validate: IContextualValidator = (stage: IStage) => {
-  const validation = buildValidators(stage);
-  validation.field('account', 'Account').required();
-  return validation.result();
+  const formValidator = new FormValidator(stage);
+  formValidator.field('account', 'Account').required();
+  return formValidator.validateForm();
 };

app/scripts/modules/core/src/pipeline/config/stages/script/ScriptStageConfig.tsx

@@ -1,7 +1,7 @@
 import * as React from 'react';
 
 import { IStageConfigProps, FormikStageConfig, IContextualValidator } from 'core/pipeline';
-import { FormikFormField, TextInput, TextAreaInput, CheckboxInput, buildValidators } from 'core/presentation';
+import { FormikFormField, TextInput, TextAreaInput, CheckboxInput, FormValidator } from 'core/presentation';
 import { HelpField } from 'core/help';
 
 export const ScriptStageConfig: React.SFC<IStageConfigProps> = stageConfigProps => (
@@ -82,8 +82,8 @@ export const ScriptStageConfig: React.SFC<IStageConfigProps> = stageConfigProps
 );
 
 export const validate: IContextualValidator = stage => {
-  const validation = buildValidators(stage);
-  validation.field('command', 'Command').required();
-  validation.field('scriptPath', 'Script Path').required();
-  return validation.result();
+  const formValidator = new FormValidator(stage);
+  formValidator.field('command', 'Command').required();
+  formValidator.field('scriptPath', 'Script Path').required();
+  return formValidator.validateForm();
 };

app/scripts/modules/core/src/pipeline/manualExecution/ManualPipelineExecutionModal.tsx

@@ -9,7 +9,7 @@ import { SubmitButton, ModalClose } from 'core/modal';
 import { Application } from 'core/application';
 import { AuthenticationService } from 'core/authentication';
 import {
-  buildValidators,
+  FormValidator,
   IModalComponentProps,
   ReactModal,
   SpinFormik,
@@ -339,8 +339,8 @@ export class ManualExecutionModal extends React.Component<IManualExecutionModalP
   };
 
   private validate = (values: IPipelineCommand): any => {
-    const validation = buildValidators(values);
-    return validation.result();
+    const formValidator = new FormValidator(values);
+    return formValidator.validateForm();
   };
 
   public render(): React.ReactElement<ManualExecutionModal> {

app/scripts/modules/core/src/presentation/forms/README.md

@@ -61,15 +61,18 @@ In addition to the `validate` prop, the field can also be validated at the form
 This example shows validation of a formik field using the `Formik` component's `validate` prop.
 
 ```js
-import { buildValidators } from 'core/presentation';
+import { FormValidator } from 'core/presentation';
 
 <Formik
   initialValues={{ email: '' }}
   validate={values => {
     const emailRegexp = /[^@]+@[^@]+/;
-    const validator = buildValidators(values);
-    validator.field('email').required([value => (emailRegexp.exec(value) ? null : 'Please enter a valid email')]);
-    return validator.result();
+    const formValidator = new FormValidator(values);
+    formValidator
+      .field('email')
+      .required()
+      .withValidators(value => (emailRegexp.exec(value) ? null : 'Please enter a valid email'));
+    return formValidator.result();
   }}
   render={formik => {
     return (