This has been a long time coming, finally found the motivation recently :)

This PR adds support for custom keywords (which are just flags). You can use these to represent labels in a webmail (#763), and sync those same labels with for example Thunderbird over IMAP.

Overview:

• Added keywords array to message CRUD
• Added endpoint to compute counters for a keyword, using redis and deltas in the same manner as mailbox counters
• Once that endpoint has been called and the value is in redis, keyword counters will be emitted over sse (KEYWORD_COUNTERS) for that keyword. Implemented it this way to avoid computing counters for irrelevant keywords IMAP clients might add
• Keywords also added to sse events related to message creates/updates. keywords on EXISTS, and addedKeywords and removedKeywords on FETCH
• Added seperate endpoint to compute counters for flagged messages. Given \flagged is a system flag it shouldn't be part of custom keywords, but knowing the total/unread for flagged messages is still very helpful
• Also added SSE for that (FLAGGED_COUNTER)
• No changes to IMAP or indexes, this was already fully supported

Some things:

• The current flag update code didn't allow for both deleting and adding flags in the same call, it would for example error if you set/unset both seen and deleted in the same api call. This is a MongoDB limitation, you can't use $addToSet and $pull in the same query. I switched it over to a simple aggregation pipeline that does allow for this.
• Setting draft set flagged in the db for some reason? Assuming that's wrong and fixed
• There were no tests for filter actions, so while I was there I added tests for all existing actions as well

I normally only work on modern TS codebases, so JS plus the old callback style isn't too familiar for me. Let me know if you see any obvious mistakes :)