Fix: Contribute: Log-in link

We were still using the old-style
log-in link (with no page destination)
on the contribute page.
Contribute: Add bubble explaining need to log in

When people have recurring contributions, we need
them to log in, so we can save them to their
account. Make a little note explaining as much.
Contribute: React if login is req for contribution

As we need people to log in before they make recurring
contributions, show as much by fading out the items
that are affected.
Refactor: Contribute: Remove tmp test code
Contribute: Handle errors from Stripe

Seems like it would be a good idea to show
reasonable error messages when our call
to Stripe's API fails.
Fix: Contribute: No need for bubble height

As text sizes and wrapping behavior is slightly
different in different browsers, getting rid
of the absolute height for the chat bubble is
helpful.