[WIP] EZP-21527: Simple search with platform stack

Link: https://jira.ez.no/browse/EZP-21527

## Status
This issue still has a lot of todos left. Feedback is welcome and if you have leads on how to solve the technical todos, feel free to comment.

## TODO
- [ ] Handle request caching
- [x] Modify the search form in the header of the page to call the new stack search. Done by @crevillo 
- [ ] Cleaner redirect on success
- [x] Add GET param from the form in the pager fanta urls (previous, next, etc)
- [ ] Find the best way to display the advanced search URL (currently a bit hackish)
- [ ] Can we display words excluded from the search ?
- [ ] Remove debug values from config files
- [ ] Rebase commits

Wondering why do we need this entity here. It's really needed?