Added new setting to sort forums by unread posts as per SALR functionality.

This involves:
New preference
Renaming existing preference newThreadsFirst to newThreadsFirstUCP (This will lose users existing preference unfortunately)
New column on the threads table marking if a thread has been looked at
Updated Forum Display Fragment to show #posts/0/blank where required
Adding upgradePreferences method to upgrade preferences when a new version installs.
Merge pull request #185 from Bonnett/sortthreads

Added new setting to sort forums by unread posts as per SALR functionality