Fix postMessage in embedded Contact form

[RoyEJohnson]

CORE-532
This makes the embedded Contact form (mostly) like the designs.

• Display just the form, not the header or mailing address sections
• After form submit, show a minimalist thank you page
• Dismissing the thank you sends a postmessage to the parent window
• Default question topic to OpenStax Assignable for the embedded form

[jivey]

Mostly small CSS concerns.

[jivey]

This is outside the scope of these changes but just wondering if we have a card to track this clipping issue on the prod errata page:

[RoyEJohnson]

This is the errata page with no book in the URL. With a book, the header gets populated and the selectors at the top are pushed down. Without a book, the page is useless.
https://openstax.org/errata/?book=Principles%20of%20Accounting%2C%20Volume%201%3A%20Financial%20Accounting

[jivey]

Do we have the book info inside this form to make the right url? Otherwise it seems like we should remove the link.

[RoyEJohnson]

D'oh! I just plugged that link in there and never thought about it.
I've updated the paragraph to direct them to the Subjects page. It's a bit cumbersome for them, but I don't think we can reliably send the book title (which is what the errata form needs).

[jivey]

We probably need some padding to push the text over or down to accommodate the close button:

(and I think this is on your assignments PR, but seems like some padding issue with that button?)

[RoyEJohnson]

I have reworked the put-away as a back button in a footer (like the design).
PR for that is:
https://github.com/openstax/assignments/pull/192

[jivey]

It might need some position: fixed rules? But I'll let ya'll work that out on that PR

[jivey]

Changes are looking good, just have a question about the errata url

[jivey]

The form is looking good, I'm still seeing some layout issues but those are on the assignments side.