Multiple thread follower rows can be added

[trichoplax]

Describe the bug
If a page is in a different state to the server (for example, if multiple tabs are open and one changes the state) it is possible to follow a thread multiple times, each time adding a separate row to the thread_followers table. Unfollowing then requires pressing "unfollow" that many times, since each time only removes 1 of the rows.

To Reproduce
Steps to reproduce the behavior:

1. Open a post with a thread under it that the user is not following.
2. Open it again in another tab.
3. In the first tab, press "follow".
4. In the second tab, press "follow".
5. There are now 2 rows in the thread_followers table for that thread and that user, and "unfollow" will need to be pressed twice before it will change to show "follow" again.

Expected behavior
There should only ever be zero or one rows in the database for a given combination of user and thread. Compare with the behaviour for following new threads on a post. See app/controllers/comments_controller lines 332-350.

[cellio]

Oops.

As part of fixing this, we should probably write a migration that cleans up any existing duplicate rows.

[trichoplax]

I'd expect it to be rare, so I'm not too concerned about cleaning up existing duplicate rows. The way this was fixed for following new threads on posts ensured that duplicate rows would make no difference to functionality, and would be automatically corrected the first time a user presses "unfollow new". I'd be happy with the same approach here. If it's decided to tidy them up in a migration anyway, we'd need to do it for both thread following and new thread following on posts.