Tidy up the custom user model

[alahiff]

Currently we query the (external) SCRC user database to obtain for each user who logs in:

• full name
• email address
• organisation(s)

If we want to store full names and email addresses of users who have logged in to the system we don't need to query the database for this as we have access to it anyway from the GitHub authentication.

Do we actually want to store email addresses and organisations? Note that this information is only available to users who are logged in.