Add link to credit card update page on expiry notifications.
Merge pull request #2471 from gittip/2471-cc-notification-link

Credit card expiration banner needs a link to setting
Merge pull request #2464 from gittip/clear_tips_receiving

Move untip.py to a method on Participant
This factors out an archive method from take_over

Right now we archive participants during take_over, but we also want to
archive participants during account cancelation.