refactor: use validator's isEmail for validating email domains

[jia1]

Problem

Currently, email domain validation is done via regex. We would like to use validator.isEmail to validate email domains. In order to use isEmail, we will prepend email domains with a string i.e. "bob".

Closes #54

Interesting things to note

I put in a lot of thought before finally deciding on "bob". I wanted something short and not nonsensical like abc.

I received these linting errors:

/Users/jiayee/FormSG/src/app/models/field/emailField.ts
  7:32  warning  Missing return type on function  @typescript-eslint/explicit-module-boundary-types

/Users/jiayee/FormSG/src/public/modules/forms/admin/directives/validate-email-domain-from-text.directive.js
  3:1  error  Parsing error: 'import' and 'export' may appear only with 'sourceType: module'

As such, I had to do the following extra stuff:

1. const EmailFieldSchema: Schema<IEmailFieldSchema> = ... instead of leaving it alone (originally const EmailFieldSchema = ...
2. const { validateEmailDomains } = require('shared/util/email-domain-validation') instead of import { ... } from '...'.

I also found out that src/app/models/agency.server.model.ts validates email domains by match: [/.+\..+/, 'Please fill a valid email domain.'],.

[liangyuanruo]

Lgtm!

[jia1]

Wait ah, I broke the build because of wrong import path. Will fix it on my end first...

[karrui]

lgtm! The second error is because you used import ... as ... syntax in a Javascript file. Our app only supports import syntax in Typescript files

[jia1]

lgtm! The second error is because you used import ... as ... syntax in a Javascript file. Our app only supports import syntax in Typescript files

Ah! I see

[karrui]

Update import to be relative before merging; tests are currently passing probably because they do not reach the invalid directive

[karrui]

lgtm