EZP-23432: As an editor, I want to be able to create content by picking a content type

JIRA: https://jira.ez.no/browse/EZP-23432

# Description

Sorry for this BIG pull requests, I was not expecting such amount of changes... And still, I created two follow up stories for [fixing/improving the content types list](https://jira.ez.no/browse/EZP-23536) and for [adding/improving the feedback to the user when she clicks *Save draft*](https://jira.ez.no/browse/EZP-23512).

For what it looks like, you can watch this screencast: https://www.youtube.com/watch?v=OnLVIHOLOlo

So now, it's possible to create a new content with the PlatformUI. For now, it will be created in English (eng-GB) and of course, like for editing, some field types are not supported but besides that, it works pretty well. Unlike in legacy, the content is actually created when the user hits "Save" or "Publish" for the first time before that, the content/version are just created client side.

# Tests

unit tests + manual test
no but introducing a constant with a name makes the code self-explanatory.