Contact Form improvements

[BenJetson]

Looks like FormSpree has better support for AJAX submissions with CAPTCHA now.

• [ ] POST the form data via XHR with fetch().
• [ ] remove target iframe for after-submission
• [ ] add our own error messages
  • [ ] on validation error
  • [ ] on receiving non-OK status from POST (allow retry)
  • [ ] on 200/OK submission, thank the user
• [ ] try to reduce spam submissions
  • [ ] change FormSpree form ID
  • [ ] find some way to obfuscate the form ID in the source code to make it harder for spambots to figure out where to send spam
• [ ] test on local box