Add possibility for mentions in the forum with '@username'

[frankystone]

Fixes #487 #CB403

Adds a new NoticeType forum_mention, that is a user get emailed if one writes in @username in a forum post. This notice type is enabled by default, but can be disabled. The content of the email send to a mentioned user is the same as for other forum notifications, except the subject is "Your name was mentioned…" and the text starts with "Your name was mentioned in a forum post by "user" on topic "topic":". To prevent sending double emails sending 'forum_mention' takes precedence over other notifications. That is:

1. For topic subscribers: for each new post in this topic containing @username, username will receive only the mail with subject "Your name was mentioned…", not "Forum new post".
2. For those who enabled "forum new topic" and his/her name was mentioned in the post of this new topic, he will receive only the mail with subject "Your name was mentioned…", not "Forum new topic".

ToDo:
[ ] javascript for a username selection box when writing @…

To get the NoticeType in run ./manage.py migrate

[bunnybot]

Mirrored on Codeberg as #CB411.