Allow users to subscribe to forum's topics
This is a new requirement for Crisis, to allow for users to subscribe to topic in order to receive notification emails when a new comment is posted.
- add global config option for the forum module to enable/disable the ability for users to subscribe to topic (default to disabled)
- a user subscribes to a topic via a new checkbox field when either creating the topic or when replying to a comment
- when a user is subscribed, they will receive an email upon new comment (vetted comment if the vetting is on) submitted on the subscribed topic