Added "confirmed" column to associates index

The account owner can see who has accepted and who hasn't accepted the
invitation.
Reinvite associates by entering in the same email address

Existing users who do not confirm their invitation in time need to be
reinvited. You can do this just by reusing their same email address on
the "Invite associate" form.
Fix associates controller spec

Adding includes(:user) to associates controller broke the associates
spec. The expectation was not being met. Just stubbed the expectation
chain.
Enable owner to assign associates to owner, user, retired

The associates index is improved with a dropdown button to change an
associate to owner, user or retired. The retired setting disables access
to the account.

The current user (which must be an owner to edit the associates) cannot
change their "ownership". They must assign a different user as owner and
let that user disable their account.