If the group's owner is spamming, unsubscribe.

This keeps the visual language consistent: rating icon plus name equals
author.

Returning null from getItem() was causing too many problems, and the
item was triggering onItemSelected() before the other items had been
loaded.

(Although it isn't possible to create an identity yet...)

The invitation and private messaging UIs are currently broken. Some key
rotation bugs were fixed; others may have been created (unit tests
needed). An encoding for private keys was added. Pseudonyms were moved
out of the messaging package and ratings were moved in.

The UI may access the DB in response to UI or DB events; to maintain a
consistent view of the DB's contents, the tasks performing these
accesses must be prevented from overlapping, and must produce consistent
results if reordered. A single-threaded executor and latches are used to
prevent tasks from overlapping, without blocking non-UI access to the
DB.