Allow intro members with accounts to use packet.csh

May require a decent bit of logic to get right, however if someone has completed their packet and received an account, they should now be able to log into packet.csh with their csh credentials and still be treated like a freshman for the purposes of signing.
LDAP should store rit username so changes shouldn't have to be made to the database schema

This is by no means a major feature requirement, but it would be really nice to see

Start by tracing the packet.csh login flow and the LDAP account lookup, then inspect how freshman status is checked for signing. Done means members who completed their packet and received an account can authenticate with their CSH credentials while retaining freshman signing treatment, with the RIT username stored in LDAP rather than requiring a database schema change.

Written by the indexing model from the issue text.