Allow to block/unblock Conversation Notifications from Settings

The user can end up in a state where the Channel of a Conversation is
blocked (has an importance of 0).
Currently there was no way to unblock it, because we didn't allow it from
the Settings. Therefore now the option to block/unblock is added to the
Conversation Settings page.

Fixes: 255062535
Test: Block/Unblock a Conversation from Settings
Test: Mark a notification blocked and conversation by clicking the “Show
notifications” toggle and the “This is a conversation” button at once.
Now unblock it from the Settings.

Change-Id: I06778d259155d87d6af4bdb7013cd3b6c9a2e5ee