Update room avatars

[baltitenger]

When matrix rooms are created for old chats, the matrix room's avatar won't get updated unless the last room avatar change happened recently enough to be in the range of backfill.

[Just-Insane]

Seconding this. There is a setting in the Instagram Mautrix bridge that does this, it would be nice if it was included in this one, not sure if the related code would be helpful.