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.

Like the plugins for the views, there's no particular class to extend to write a plugin for the app (except the YUI Plugin Base class of course). The app plugins added in this PR mostly deal with *side* tasks related to the application but that don't really belong to the application responsibility. In addition, a typical app plugin would add some routes and views to the application.

Note: this PR is done against the topic branch of #99.

# Tasks

* [x] Inject the registered plugins in the app
* [x] Refactor some app sub part as plugins
  * [x] Handlebars partial registration
  * [x] Handlebars helpers registration
  * [x] DOM state handling (minimized discovery bar, navigation mode, ...)
  * [x] ~~Dynamic app extension (like the *admin* part) ?~~ more thinking is needed on that topic, will maybe be handled later

# Tests

manual + unit tests