[WIP] EZP-23147: As a developer, I want to be able to write plugins for the apps

JIRA: https://jira.ez.no/browse/EZP-23147

# Description

This PR is a follow up of #98 ([EZP-23012](https://jira.ez.no/browse/EZP-23012)) / #99 which allows the developer to write plugins for the applications provided by the PlatformUIBundle. Actually, it is more about refactoring existing code into plugins for the app which has the benefits of dividing the currently fat application into smaller components.

# TODO

* [x] Inject the registered plugins in the app
* [ ] Refactor some app sub part as plugins
  * [ ] Handlebars partial registration
  * [ ] Handlebars helpers registration
  * [ ] Dynamic app extension (like the *admin* part) ?
  * [ ] ...

# Tests

manual + unit tests