PIM-3364 : normalize form handlers

Move some custom logic from handler to manager (schedule completeness when save family or channel should be applied on each family or channel save, not only from enrich mode).

Introduce a base handler interface and related base implementation which can be used for most of our entities, this handler relies on a SaverInterface to save the entities once the form binded and validated.

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | N
| BC breaks?           | Y
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-3364
| DB schema updated?   |
| Migration script?    |
| Doc PR               |