[WIP] [Form] [TwigBridge] Bootstrap layout whitespace control

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

While working with Symfony's Bootstrap layout I have found few minor issues:

- [x] Choice labels are rendered with extra spaces
- [ ] User's label classes are glued to `required` without any separator (without space)
- [ ] Whitespace control for all form widgets (like in default layout)

As all issues are tightly related, I think they could be fixed with this single PR.

What was done:
- All failing tests from `AbstractLayoutTest` were copied to `AbstractBootstrap3LayoutTest` and adapted for Bootstrap layout. One of the main fixes was to change class checking for input (e.g., Bootstrap requires "form-control").
- Removed spaces from `form_widget_simple` and `checkbox_radio_label ` blocks

What's left:
- Fix tests for parts where `datetime_widget` is used. There is `&nbsp;` in the template. It would be best to get rid of it.
- Trim all unnecessary spaces from widget blocks (whitespace control)

Work in progress.