[DEV-2155] Add a date_joined field to AskbotUser.
[DEV-2155] Add migration for new date_joined field.
[DEV-2155] Refactor migration to introspect Activity table.
Merge pull request #55 from roverdotcom/DEV-2155

[DEV-2155] Add a date_joined field to AskbotUser for use in SQL queries.