[Feature Request] Support for updating Matrix messages when RSS feed content changes

[eiclu]

Currently, when authors update the content of a blog post, those changes are not reflected in the messages already posted to Matrix.

To ensure consistency, Matrix-Hookshot should monitor RSS feeds for updates to previously published posts that still exist in the feed and edit the corresponding Matrix messages to reflect those changes

[Half-Shot]

Sounds reasonable 👍 . Right now we send these posts as fire and forget, so we'll need to track the event IDs of each message and remember to update them.

Basically boils down to:

1. Adding an interface to our storage layer to store event ID <-> RSS info.
2. Adding the implementations of that interface to Redis/Memory.
3. Ensuring the FeedConnection class does the storage of event changes.
4. Ensuring the FeedReader logic forwards updates of seen posts.
5. Ensuring FeedConnection handles these updates and sends new update message appropriately.