Provide email notification when MR thread is resolved

Problem to Solve

When you're reviewing many merge requests it can be challenging to keep track of threads you've opened to ensure they're resolved as you wish. Many times, threads can be resolved automatically or may be resolved by other users. In some cases, this may be fine, but in other cases you may want to take a closer look.

However, when threads are resolved, there's no notification that the event has taken place, and you're unable to then easily find and track those conversations.

Proposal

When any thread is resolved in an MR, all participants should receive an email notification.

Possible future enhancement (to be discussed)

When a thread that you have started is resolved (by someone other than you) there should be a To-Do or Email notification generated for the person who opened the thread. This allows you to know that the thread has been resolved and check in on it.