[mms] Abstract all UIDs used for mailbox/message navigation on the browser to BUIDs (browser UIDs) (Bug #10719).

Separates backend message manipulation from UI presentation.

Allows for easier/cleaner manipulation on the browser side:

  * No need to keep track of the both the current display mailbox AND the
    message mailbox - makes javascript much cleaner
  * Browser UIDs are always integers so large ranges can be efficiently
    packed using message sequence strings
  * All message manipulation is now abstracted and looks the same in
    every mailbox - no need to change method of sending UIDs based on
    the current mailbox type ("real" mailbox vs. virtual)