Twitter -> Matrix: Read receipts and typing indicators

[PurpShell]

Adding Twitter -> Matrix read receipts is paused because it involves adding support metadata fields in Portals and Ghosts (see mautrix/go#295).

Meanwhile, adding Twitter -> Matrix typing indicators requires that we listen to a SSE request. The problem is knowing which chats to listen to. This can possibly be solved by subscribing to the events for X seconds after a message is sent, but is considered resource inefficient and unpredictable.