Add trac-admin session promote command

[Ryan J Ollos]

Users aren't displayed in the ticket assign-to field (TracTickets#Assign-toasDrop-DownList) until they have authenticated with the project ([ticket] restrict_owner = true). Some users choose to run a SQL query on the database to insert "known users". To help with this recurring issue we could extend the session set command or add a session promote command, either of which would effectively execute Select.promote_session​.