Show success message after submission of feedback form.

[drcongo]

Finishing touches for #271

Replace the feedback form with a success message after it has been submitted.

Thank you for letting us know. This helps us understand our audience, so we can make our resources more useful.

• [x] The feedback form should no longer be in the rendered template when we return to the page after submitting
• [x] The user should be able to dismiss the success message