Unify threads views/forms/template dirs/url names to follow `[Private]ThreadVerb{Type} format`

[rafalp]

I am getting annoyed at how messy this stuff becomes. I'll clean it up after I finish doing features.

reply_private_thread -> private_thread_reply HidePrivateThreadUpdate -> PrivateThreadHideUpdate unread-thread-post -> thread-unread-post

[rafalp]

Keeping things consistent in such a large codebase is becoming pain. Guess I should create a naming reference in doc.

[rafalp]

Dev guide is coming with #1977 that will be first feature to follow it.