chore(infrastructure): Report screenshot test results on GitHub PR

[acdvorak]

What it does

• Posts screenshot test statuses and diff report links on every PR
  • The "Details" link takes you to the Travis CI job A) while the test is running, and B) when it fails with an error
  • The "Details" link takes you to the report page when the screenshot test completes
  • Uses the GItHub commit status API and a bot account: @mdc-web-bot
• Adds an encrypted credential file for the GitHub API
• Adds color and collapsible sections to Travis CI log output to make it easier to read

Notes:

• Travis CI screenshot tests currently diff against the PR's golden.json file, so the tests will pass if the browsers render what the PR says they do. It's still up to the PR author and reviewer to diff against origin/master's golden.json and make sure the change makes sense.
• We should create a follow up PR that links to a diff against master's golden.json instead.

How to test it

1. Create a PR that modifies a file in packages/ or test/screenshot/ (such as this one).
2. Scroll to the bottom of the PR in GItHub.
3. After about 2 minutes, a screenshot-test/butter-bot status should appear - like magic.

Example output

Pending - Running screenshot tests:

Passed - No diffs:

• Travis job
• Diff report

Failed - 4 Diffs found:

• Travis job
• Diff report

Full page:

[codecov-io]

Codecov Report

Merging #3080 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #3080   +/-   ##
======================================
  Coverage    98.3%   98.3%           
======================================
  Files         101     101           
  Lines        4368    4368           
  Branches      564     564           
======================================
  Hits         4294    4294           
  Misses         74      74

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