create coop scaffold. add user-coop db relationship
redirect user to user edit page on account creation (first sign in) where they can set their username and coop
when user specifies a coop, the id of the coop with the given name is put in the user model. if the coop doesn't exist yet it is created with the coop name the user specified.
Merge pull request #4 from daf-/co-op-calendar

coops and user account edit on creation