Abstract preview caching code into separate object
Handle preview FIFO determination within a single object
Bump preview cache size
Just in case this is implemented, don't make it not work on browsers going forward
Abstract out sending the list of preview UID tuples

Idea going forward: for search mailboxes, we will be able to save a
single preview cache object for the search mailbox UID and the real
mailbox UID.
Fix updating messages when applying status actions and the part content is HTML data
Much improved population of addresses into compose header fields

Always use the autocomplete code to insert - this ensures that 1) we
aren't trying to parse e-mails as much as possible on the browser and 2)
take advantage of the fact that the server provides better display
information (such as improved short labels and group processing).
Fix updating hidden AC input value when resetting a field
AC focus fix