Improve performance of recipient generator #508
Description
Right now, the recipient generator queries the entire list of event objects just to build a set of user IDs. Which is quite time- and memory-inefficient, especially considering requests that could have hundreds or thousands of comments and events.
We should improve this by either:
- Using a more efficient aggregation query
- Maintaining a separate "watchlist" of users associated to a request. Users would be automatically opted in when they participate in the request in any way, but can opt out or opt in manually. Similar to how GitHub's issue notifications work.