Review migrated/new users distinction

[leggsimon]

If I recall correctly, migrated users was used as a way to give clients access to the new syndication platform when their contract was renewed or something. I'm not sure if we need to have any concept of "migrated users" now since everyone is migrated and any new users are just new users, I'm not sure why they get handled hugely differently.

I think we should review the usage of:

• FT-New-Syndication-User header (possibly used in n-syndication)
• res.locals.isNewSyndicationUser usage
• the entire migrated_users table
• the syndication.get_migrated_user() SQL function

#69 removed some code around "migrated users"