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.
Merge pull request #2465 from gittip/factor-out-archive

Factor out archive
Smooth out take_over tests

Change from `alice_participant` and `alice` to `alice` and
`alice_twitter`. Makes it easier to reason about participants vs.
accounts elsewhere if we use this nomenclature.
Merge pull request #2466 from gittip/smoother-tests

Smoother tests
This test gives us 100% coverage of take_over

That doesn't mean there aren't any bugs, of course. :-(
Merge pull request #2467 from gittip/cover-take_over

This test gives us 100% coverage of take_over
Refactor reserving a username

There's actually a bug in this, we get an InternalError when the first
username attempt is taken and we try a second one. I don't have time to
debug that right now. At least the code is deduped now.