Bugzilla

We should update our message list to set some sane default.
- Threads should be enabled by default in the Inbox folder (other folders might need different setting but a non threaded Inbox is very strange).
- Sorting should be by date descending (from newest to oldest).

These changes shouldn't override users' edited settings, so only change the defaults but not override previous changes.

We should update our message list to be threaded by default.
This change should only be available for new profiles, therefore a migration is needed to change this pref without affecting existing profiles or users that changed that pref already.