Add handler for profile changes

[AndrewFerr]

This causes Signal profile changes to be fetched as they happen, but only for logged-in users.

Helps with #476

[tulir]

Signal actually sends events for profile changes? I couldn't see anything when I last tried it 🤔 (or is this only for the logged-in user?)

[AndrewFerr]

Ah, it's only for the logged-in user, good catch.