When rendering a label we want to check 'scoped_label' feature
availability on a project/group where label is being used. For
this reason a label presenter is used in UI and information about
context project/group is passed to this presenter.
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.
