Some URL shortening services support user accounts. Each shortening service works in one of three ways:

• open services which don't have any user accounts, such as is.gd
• closed services which only allow authenticated users, such as dznr and (likely many) private services
• hybrid services which allow both auth'd and non-auth'd accounts, such as bit.ly

The purpose of this ticket is to add optional support for user accounts for any services which support them, exclusively for shortening (expanding URLs should work for all services, regardless of user accounts). Since some services require them, if the service does not have a user account associated, it should not be considered valid. Credentials should be stored within the Keychain.