Introduced the light entity form type

```
Bug fix: no
Feature addition: no
Backwards compatibility break: yes
Unit test passes: yes
Specs passes: yes
Behat scenario passes: ?
Checkstyle issues: no
ChangeLog updated: no
Documentation PR:
Fixes the following jira:
```

It allows to provide choice list of entities without hydrating all the
entities of the collection. It delegates the choices list generation to
the entity repository which then can provide an array of id => label.