https://assembly.com/coderwall/bounties/455

Currently the welcome email checks if @user.on_team? and then if they are presents some info referencing @user.team https://github.com/assemblymade/coderwall/blob/master/app/views/notifier_mailer/welcome_email.html.haml#L20
User#on_team? previously checked for team_document_id. I talked with @seuros who told me that team_document_id is deprecated and will be removed, and that we're temporarily using team_id until support for belonging to multiple teams is added.

I've updated User#on_team? to check if a user has a team_id or a membership. I also updated User#team so that it returns the team associated with the membership (but also still supports associating teams by team_id).

This is my first PR on this project and this is one of the larger, more complex projects I've worked on so let me know if there is anything I should be doing differently.