feature #33954 Form theme: support Bootstrap 4 custom switches (romaricdrigon)

This PR was merged into the 4.4 branch.

Discussion
----------

Form theme: support Bootstrap 4 custom switches

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | yes
| Deprecations? | no
| Tickets       | /
| License       | MIT
| Doc PR        | symfony/symfony-docs#12464

Hello,

At the moment, Symfony form theme supports [custom checkboxes](https://getbootstrap.com/docs/4.3/components/forms/#checkboxes) through an extra class in `label_attr`.
Bootstrap4 introduced also [custom switches](https://getbootstrap.com/docs/4.3/components/forms/#switches), which has exactly the same HTML markup, but use a different class. This PR slightly modify `bootstrap_4_layout` to handle it.

![image](https://user-images.githubusercontent.com/919405/66651725-0eaa3100-ec34-11e9-8b68-94324730ac80.png)

Some reasons why I think supporting those have its place in Symfony:
 - those are getting common in UI right now, it is a common use case
 - it is complementary to normal checkboxes, and works the same way: required attribute, validation error, and so on are supported immediately
 - implementing it yourself in your form theme is actually tricky, because of the way checkbox are handled (ie., `form_label` called inside `form_widget` with a `{ widget: parent() }`). You have to overwrite the whole fragment, otherwise you get an infinite recursion.

Finally, some screenshots and code examples.

Custom checkbox (as at the moment):
![image](https://user-images.githubusercontent.com/919405/66652982-41a1f400-ec37-11e9-813f-4b39087e89e7.png)
```php
    ->add('test', CheckboxType::class, [
        'label_attr' => [
            'class' => 'checkbox-custom',
        ],
    ])
```
Custom switch (proposed):
![image](https://user-images.githubusercontent.com/919405/66652902-1919fa00-ec37-11e9-98f3-9340b01b2335.png)
```php
    ->add('test', CheckboxType::class, [
        'label_attr' => [
            'class' => 'switch-custom',
        ],
    ])
```

Commits
-------

99f59e262f Supporting Bootstrap 4 custom switches

Author: fabpot

CHANGELOG.md

@@ -10,6 +10,7 @@ CHANGELOG
  * the `LintCommand` lints all the templates stored in all configured Twig paths if none argument is provided
  * deprecated accepting STDIN implicitly when using the `lint:twig` command, use `lint:twig -` (append a dash) instead to make it explicit.
  * added `--show-deprecations` option to the `lint:twig` command
+ * added support for Bootstrap4 switches, use `switch-custom` as `label_attr` in a `CheckboxType`
 
 4.3.0
 -----

Resources/views/Form/bootstrap_4_layout.html.twig

@@ -166,6 +166,11 @@
         <div class="custom-control custom-checkbox{{ 'checkbox-inline' in parent_label_class ? ' custom-control-inline' }}">
             {{- form_label(form, null, { widget: parent() }) -}}
         </div>
+    {%- elseif 'switch-custom' in parent_label_class -%}
+        {%- set attr = attr|merge({class: (attr.class|default('') ~ ' custom-control-input')|trim}) -%}
+        <div class="custom-control custom-switch{{ 'switch-inline' in parent_label_class ? ' custom-control-inline' }}">
+            {{- form_label(form, null, { widget: parent() }) -}}
+        </div>
     {%- else -%}
         {%- set attr = attr|merge({class: (attr.class|default('') ~ ' form-check-input')|trim}) -%}
         <div class="form-check{{ 'checkbox-inline' in parent_label_class ? ' form-check-inline' }}">
@@ -238,7 +243,7 @@
     {#- Do not display the label if widget is not defined in order to prevent double label rendering -#}
     {%- if widget is defined -%}
         {% set is_parent_custom = parent_label_class is defined and ('checkbox-custom' in parent_label_class or 'radio-custom' in parent_label_class) %}
-        {% set is_custom = label_attr.class is defined and ('checkbox-custom' in label_attr.class or 'radio-custom' in label_attr.class) %}
+        {% set is_custom = label_attr.class is defined and ('checkbox-custom' in label_attr.class or 'radio-custom' in label_attr.class or 'switch-custom' in label_attr.class) %}
         {%- if is_parent_custom or is_custom -%}
             {%- set label_attr = label_attr|merge({class: (label_attr.class|default('') ~ ' custom-control-label')|trim}) -%}
         {%- else %}

Resources/views/Form/bootstrap_base_layout.html.twig

@@ -172,7 +172,7 @@
 
 {% block choice_label -%}
     {# remove the checkbox-inline and radio-inline class, it's only useful for embed labels #}
-    {%- set label_attr = label_attr|merge({class: label_attr.class|default('')|replace({'checkbox-inline': '', 'radio-inline': '', 'checkbox-custom': '', 'radio-custom': ''})|trim}) -%}
+    {%- set label_attr = label_attr|merge({class: label_attr.class|default('')|replace({'checkbox-inline': '', 'radio-inline': '', 'checkbox-custom': '', 'radio-custom': '', 'switch-custom': ''})|trim}) -%}
     {{- block('form_label') -}}
 {% endblock choice_label %}