Code improvements, bug fixes, finish documentation and specs

This commit adds a number of _html columns and, with the exception of Note,
starts updating them whenever the content of their partner fields changes.

Note has a collision with the note_html attr_accessor; that will be fixed later

A background worker for clearing these cache columns is also introduced - use
`rake cache:clear` to set it off. You can clear the database or Redis caches
separately by running `rake cache:clear:db` or `rake cache:clear:redis`,
respectively.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Projects::ForkService delegates to this service almost entirely, but needed
one small change so it would propagate create errors correctly.

CreateService#execute needs significant refactoring; it is now right at the
complexity limit set by Rubocop. I avoided doing so in this commit to keep the
diff as small as possible.

Several tests depend on the insecure behaviour of ForkService, so fi them up at
the same time.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Closes #17348

The feature spec now also tests the absence of the new branch button