Project activities perfomance increased. Cache for project activitites disabled. Repository conception changed

Allows to alert only the commit author when a new note is added on a commit, useful when gitlabhq is used for code
reviews, allows less noise with mails...

This reverts commit f80745b7.

Removes destroy callback for users_projects when a project is destroyed.
This works okay becaues the gitolite config will be updated at at the
end to remove the entry entirely, thereby removing the users anyway.