Changing a message's category now takes effect immediately.

- Picking "Change category" on a message added the rule but left the message where it was until you re-sorted the whole inbox by hand. A rule tied to a sender - which is what that option creates - is now applied to that sender's existing messages straight away. Closes #1 
- Rules that match on the message text instead of a sender could apply to anything, so they are not applied unbidden: the app says so, and offers to take you to Settings, scrolled to "Sort inbox again" with the option briefly highlighted. New messages use such rules right away regardless.
