Un-subscribing from an issue does not result in Todos being "muted"

This is based on what I'm seeing at GitLab.com, the behaviour appears to be as following:

1. Somebody mentions you in a issue/merge request, you're now subscribed to the thread
2. You un-subscribe from the thread via the web interface
3. Somebody mentions the issue or edits it
4. You still get a todo, even if you're not explicitly mentioned

cc @dbalexandre