Fixed "bug" computing next billing date.

All our Authorize.Net subscriptions are set to recur every 365 days, so
our next billing date has to reflect that instead of a full year.
Fixed off-by-one-sometimes bug in "days to rebill."

It was computed by comparing a semi-imaginary "start date/time" with the
current moment in time, and then rounding down to get a number of days.
That meant if you signed up and then looked at your billing page, your
next billing date would be in about 23 hours, which we would call 0
days. Now we compare the current *date* with your start *date*, and call
it 1 day, i.e., you'll be charged tomorrow.
Next billing date: show both date and "days until."

https://www.pivotaltracker.com/story/show/67956928
Fixed setting start_date to a date instead of a time.

We basically want it to be a date, but for unknown historical reasons,
the database column is a datetime, so let's keep it that way for now.

Also removed some specs for "days until rebill for a subscription with
no start date." We never display this information for a subscription
with no start date, and 0 isn't an intuitive answer (it's more like
NaN), so let's leave it unspecified for now.

https://www.pivotaltracker.com/story/show/67956928
Set next billing date on notification of payment.

https://www.pivotaltracker.com/story/show/67956928
Fixed payment factory spec.

Made a last-minute change in the previous commit, forgot to update the
test.
Fixed/removed some pending specs.

Or in one case just determined it didn't need to be pending, because the
spec was fine.
Manually update next billing date.
Let admins extend subscription when entering payment.

https://www.pivotaltracker.com/story/show/67956928
Refactor: Started extracting "test page" class.

Eventually I think we'll move some of the form-filling functionality to
a superclass, too.

Temporarily removed "extend subscription by 365 days" test; it'll be
added back as part of payment by check.
Pay by check: Input check number.

To do: make it possible to have a next billing date for an account
with no Authorize.Net subscription.

https://www.pivotaltracker.com/story/show/67958188
Let pay-by-check orgs have a next billing date.

Moved next billing date from Subscription to Organization, so
everyone can have one.

https://www.pivotaltracker.com/story/show/67958188
More consistent admin headings/navigation.

* All of the per-organization admin pages (including users, billing,
  payment history) now remind you what org you're editing.
* They don't tell you what you're doing, because you can figure it
  out from the tabs and the content.
* They all use the same navigation tabs.
* Pages that show org actions (e.g., cancel billing) now all show the
  same actions.