Improve select2 configuration

This PR improve the select2 configuration.

To disable select2 on some `select` form element, set CSS class `select2-disabled` to this form element.
To overload default functionality of `allowClear`, set CSS class ``select2-allow-clear-enabled`` to enable ``allowClear`` or ``select2-allow-clear-disabled`` to disable ``allowClear`` parameter.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1889, #1776
| License       | MIT