Subscribe to forum

[Azareal]

We want users to be able to subscribe to forums, there are several use cases for this:

• The user might want to be notified when an announcement comes in.
• They might want to be informed when a report is made. Reports are currently implemented with a reports forum, so this is one way to do moderation alerts without necessarily having to implement moderation alerts, although it might be useful to have a second alert stream to seperate it from the more mundane things in the header.
• A user might want to be informed of everything that happens on a small site without much activity.
• A user might want to be informed of everything that happens in a small forum without much activity.