Wiki: Rewrite the current Wiki form implementations for create/edit from HAML to Vue

What to do

As prerequisite to implement the upcoming RTE (Rich Text Editor) we need to port the current Edit and Create Experience over from HAML to Vue. In this iteration it's not expected to add more functionality than there is already, but the experience needs to be at least on-par with the current one. This means there's also no need to right now to change how the form gets submitted, unless easily doable as APIs exist.

Attention Lets focus on the normal Wiki for now. Porting the same functionlity to GroupWiki and come in a seperate iteration.

Assumptions

Backend REST endpoints are there so the experience can be ported without the need of the frontend.

Success Criteria

• There's a feature flag available behind which the new Vue Wiki Edit/Create experience is reachable
• The Vue implementation is at least on-par with the current one
• VTU specs verifying the behavior of the wiki edit experience

Availability and Testing

Existing tests will need to be updated, please involve a SET in MR reviews. Any changes not behind a feature flag will require package-and-qa job to pass for regression purposes.