Clean up mw.util.jsMessage

Clean up mw.util.jsMessage

* Re-use hideMessage()
* Actually set messageBoxEvents to true so that the one-time
  setup is only set up once.
* Optimize hex colors ({6}->{3}).
  While at it, also ran it for the rest of the file.

* Follows-up I41c70d78

Change-Id: I1863b62003edc825fdefe53d02c2d22e36409234