Donation support form

[Sancus]

We need a Thunderbird version of the form we currently link to here: https://donate.mozilla.org/en-US/help/

In order to do this we need to figure out how to set up a form that feeds into SalesForce. Presumably it has embeddable forms or some sort of API that can be called by JavaScript to feed support requests in.

Worst case scenario we can do a server side piece that uses the email address, but that's less ideal. We should first determine if we can avoid that.

[MelissaAutumn]

Foundation uses form assembly, we'll get a copy of the embeddable html form (with recaptcha support) either tomorrow or friday.

[MelissaAutumn]

Oof that html did not come cleanly. I threw away all of their styling and re-wrote it. I need to confirm that we have the final remote urls (for the js library) before putting a PR up. But it works, and is mostly pretty.

Here's the outstanding work:

• [x] Confirm remote urls
• [ ] Get a style check (@micahilbery 😄?)
• [ ] Push up new strings to l10n repo (once approved)
• [x] Figure out if we can have a customized Thank you page (Right now it's just a tiny "Thank you" on a off-white page.)
• [ ] Adjust the "Contact Us" link on FRU

One annoying limitation is that FA doesn't handle any form errors on this page, and instead sends you to the unstyled form on their site. I've added a required field to all of the inputs just to avoid that weird case.