Feature: More sophisticated handling of forked PRs in landers

[ryan-roemer]

Our landers (e.g. spectacle/docs) get forked PRs against the docs. But, Travis does not provide encrypted secrets for forked PRs for security reasons.

• Backstop: Should not fail CI for invalid credentials.
• Figure out if we can still do staging deploys?
• Otherwise, figure out if we can somehow do a PR message?
  • CI specific feature? (Like Travis CI has special way to post comment or something)
  • GitHub actions? (Note: GH token is available in forks, but it is a read-only token: https://www.edwardthomson.com/blog/github_actions_11_secrets.html)
  • (UGH, ticket) a lambda webhook (like we've done before)