[WIP] PIM-3364 : normalize form handlers

Move some custom logic from handler to manager (schedule completeness when save family or channel).
Introduce a base handler interface and related base implementation which can be used for most of entities.

| Q                    | A
| -------------------- | ---
| Bug fix?             |
| New feature?         |
| BC breaks?           |
| CI currently passes? |
| Tests pass?          |
| Scenarios pass?      |
| Checkstyle issues?*  |
| PMD issues?**        |
| Changelog updated?   |
| Fixed tickets        |
| DB schema updated?   |
| Migration script?    |
| Doc PR               |