Matrix notifier for questions and announcements

This is similar to the Telegram bot that was recently upstreamed. We use Matrix for our internal communications, so this basically has the same functionality but for Matrix. I think this is likely to be valuable to others, seeing as how Matrix has (at least at some point) been used for official IOI tasks.

Our implementation (https://github.com/e-i-o/cms/commit/32f90fce305e0d553e73f510716d92c3997f10f1) is like one of the old Telegram bot prototypes, in that it's write-only and lives directly in the CWS process. It would probably be possible to make it read-write, allowing to reply directly from matrix, but it would be a lot more effort and probably necessitate pulling in a matrix client SDK. It could also be moved to a separate Service while remaining write-only, which addresses some concerns I saw regarding the old telegram bot (means that CWS doesn't need to make web requests).

Start by comparing the existing Telegram bot with the linked implementation commit and inspect how the CWS process handles notifications. Decide whether the Matrix notifier should be write-only in CWS or a separate service, and clarify the expected question and announcement behavior before implementation. Done means the chosen Matrix integration works for those notifications.