max-users raised on HDBits

Raised the user limit to 20,000 to make the number more round, and to let those people who have been after invites for a while in.

Unfortunately we've had to remove invites from users who are 720p or below .. as the site fills up fast and this is a little bonus to our users who seed well smilie

HDBits has been rather full for a few days/weeks, now it looks like a batch of fresh users is able to join - through invites only, obviously. Just thought I'd let those know who are dying to get in. About 1950 slots are still free.