WRT posting dashboard should use the ticket status instead of `posting_by` column

[danieljames-dj]

The posting_by column is no longer needed in competitions table. This was used to lock the competition for results posting. But now since all the results are connected to tickets, we can actually use the ticket's status to show whether the ticket is locked or not.

This way, even if WRT uploads the results JSON later through admin page, the competition can be locked through the posting dashboard.