Report the latest feed entry when fetching a feed for the first time

[tadzik]

We only do this for persistent storage, otherwise we'd trigger a wave of spam on every restart.

TODO:

• [ ] Actually fetch the feed when we create a connection instead of waiting for the pool queue to catch up.

Fixes https://github.com/matrix-org/matrix-hookshot/issues/806

On second thought, that won't work just yet – this will only report the newest entry for brand new feeds that no one has subscribed to before. We'll need some special path to make it possible to repost a previously seen entry if a room is new.