Add building of subscriptions table data.

Some users have subscribed without having commented or voted.  This
added additional complexities as subscription db in mongo lists users by
external_id unlike all other tables which list the username... oh crap.

Refactored things around considerably.  Mongo comments and subscriptions
are now loaded up front, so all following procedures can operate on
them.  Added lots of messy logic into UsersTable, building an additional
index by external_id and looking up usernames from Sencha Forum db by
external_id. Thankfully it's a throw-away script, as the resulting code
is quite fragile and messy.

Also found one user who had subscribed bot no more has an entry in
Sencha Forum database.  Going to throw away his 3 subscriptions.