Minimal thread views

[rafalp]

Adds minimal thread views that can be later completed with missing features in smaller PRs

Part of the #1774 epic

TODO

• Generic thread view
• Generic private thread view
• Minimal thread view
  • Check see permissions
  • Redirect if slug is outdated
  • Check posts permissions
  • Paginated posts list
  • Redirect if page is invalid
• Minimal private thread view
  • Check see permissions
  • Redirect if slug is outdated
  • Check posts permissions
  • Paginated posts list
  • Redirect if page is invalid
• Plugin hooks
• Template plugin outlets
• Update hooks reference

[rafalp]

This PR will break tests. Fixing those will take a while because some of them rely on features not yet present in new thread views.

[rafalp]

I've deleted bunch of test suites for previous implementation of this view. Only tests failing now are from redirects views. I'll merge this when I finish the todo, then implement new redirect views using new permissions and plugins. That should fix our tests suite and leave me to work on next stuff.