(feat): display that another user has blocked them and give them the option to block them back

When a user is blocked and the blockee tries to visit the blocker's channel, don't tell them the channel couldn't be loaded. Tell them "This channel (@username) has blocked you." and show the block/unblock button beneath it so they can block them back if they want to. This will let users know whether a channel has been deleted/deactivated or blocked them and will prevent a common troll tactic.