Handle UI delete in EAS where the message is moved to trash

* Check for the folder of updated messages, and delete those now in trash
* Created convenience method for finding a mailbox of a particular type in
  an account (helpful for finding, say, an Inbox or Trash for a given account
* Fix deleted/updated message table so that the _id column is defined to be
  unique (to prevent duplicate change records from being written)
3 files changed