[WIP] EZP-22654: Section create and edit

Link: https://jira.ez.no/browse/EZP-22654

## Description
Adds section create and edit.
It also fixes a few small issue  and CS.
I refactored two things:
- The way section list were returned (it now returns an array of objects instead of an array of array)
- I created a dedicated action to forward to when access is denied to a user (in PjaxController).

## Tests
Manual tests

## TODO
- [ ] Apply PjaxController accessDenied to other controller than section.

*The section identifier can not be empty.*