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

Hi, I've added a new setting to sort forums by unread posts (like in SALR) - it's separate to the UCP sorting cause goons like to moan

This involved:
New preference
Renaming existing preference newThreadsFirst to newThreadsFirstUCP (This will lose users existing preference unfortunately, I can't see how to get around this without leaving confusing variable names)
New column on the threads table marking if a thread has been looked at - I've updated the create & onUpgrade code plus the DB Version
Updated Forum Display Fragment to show #posts/0/blank where required

Cheers,

JingleBells