New feature: add 'Mention' notification level

It does disable all emails expect system one or when you was @mentioned

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add mention notification level to CHANGELOG

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'mention-notification-level' into 'master'

Mention notification level

It does disable all emails expect system one or when you was @mentioned.

Fixes #1724

See merge request !1359
Merge branch 'master' of github.com:gitlabhq/gitlabhq