Add ability to make the <SaveButton> not disabled by default

[Luwangel]

Implements #4913

To build this feature I allow the user to pass a new prop named alwaysEnableSaveButton to the <Toolbar> which forces enabling ou disabling the <SaveButton>. If the user doesn't specify this prop, we keep the classic behaviour by disabling the button until the form is dirty.

Todo

• Allow the <SaveButton> to be disabled by passing the disabled prop
• Create a new prop called alwaysEnableSaveButton in the <Toolbar> and implements its mechanism
• Keep the condition pristine equals to disabled only if the alwaysEnableSaveButton prop has not been explicitly passed to the <Toolbar> but move the logic to the <Toolbar> instead of the <SaveButton> (possible breaking change here)
• Fix and improve the documentation
• Fix tests

Example

How to force enabling the <SaveButton> without rewriting the whole <Toolbar>:

import * as React from 'react';
import {
    Create,
    SimpleForm,
    Toolbar,
} from 'react-admin';
import PostReferenceInput from './PostReferenceInput';

export const CommentCreate = props => (
    <Create {...props}>
        <SimpleForm redirect={false} toolbar={<Toolbar alwaysEnableSaveButton />}>
            // Form goes here
        </SimpleForm>
    </Create>
);

If you don't pass the disabled prop, the behaviour "pristine equals to disabled" will be applied as before.

Screenshots

The following screenshots are outdated because the prop has changed. You need to use alwaysEnableSaveButton instead of disabled and invert the condition.

Force enabling the <SaveButton>

Force disabling the <SaveButton>

Keep the pristine equals to disabled mechanism

[fzaninotto]

Why is this a breaking change?

[Luwangel]

Why is this a breaking change?

People who use directly the <SaveButton> cannot rely on the pristine prop anymore to disable the button. They have to use the disabled property instead. It's not a breaking change if they use the <SaveButton> inside the <Toolbar> or the <SimpleForm>.

So most of the time it's not a breaking change. But for some isolated cases, it could be one minor breaking change. It's better to mention it just in case.