This change adds three new filters to the threads API. They are:
* Filtering only threads that are flagged for abuse
* Filtering by the thread type (discussion or question)
* Filtering by the thread author

In addition it also adds a new ``abuse_flagged_count`` field for threads. It
returns a count of the number of comments in a thread that are flagged for abuse.
This is only visible to users that have moderator privileges or higher.

Finally it also adds a ``abuse_flagged_any_user`` field that is set if any user
has flagged a thread. This field too, is only visible to moderators or above.

Co-authored-by: Kshitij Sobti <kshitij@opencraft.com>