Add teams model
Add team spec
Add polymorphism to memberships and fix everything so specs pass again
Users can add members to teams
User can add members to teams + test coverage
Add team invitation mailer and spec coverage
Users can accept team invitations via email
Inviters and owners receive email when users join their projects
Add team invitation accepted mailer spec and update project invitation accepted mailer spec
Users can accept team invitations via homepage and spec coverage
Users can decline team invitations via homepage and spec coverage
Team owners can remove members
Team owners cannot remove themselves from projects accidentally