Add trac-admin session promote command

[Ryan J Ollos]

When [ticket] restrict_owner = true, users aren't displayed in the ticket assign-to field (TracTickets#Assign-toasDrop-DownList) until they have authenticated with the project. As a workround, some admins 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 or directly execute Session.promote_session​.

The issue was recently raised again in ​gmessage:trac-users:vKI_MDGL-Fs/EkAu9fiNCQAJ.