chore(infrastructure): Add npm run lint:html for screenshot pages

[acdvorak]

What it does

• Adds npm run lint:html command
  • Runs against test/screenshot/spec/**/*.html files (excluding index.html)
• Adds root .htmllintrc config file
  • Unfortunately, htmllint doesn't support disabling individual rules with inline comments.
  • However, you can disable all rules with a comment block:

    <a>Allowed tag</a>
    
    <!-- htmllint-disable -->
    <b>Banned tag</b>
    <!-- htmllint-enable -->
    
    <code>Allowed tag</code>

• Fixes linter errors in button, card, drawer, radio, switch, and textfield pages'
• Adds <label> elements to radio and switch pages:
  
  

How to test

npm run lint:html

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@cd28c19). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #3502   +/-   ##
========================================
  Coverage          ?   98.4%           
========================================
  Files             ?     119           
  Lines             ?    5028           
  Branches          ?     618           
========================================
  Hits              ?    4948           
  Misses            ?      80           
  Partials          ?       0

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[mdc-web-bot]

🤖 Beep boop!

Screenshot test report 🚦

20 screenshots changed from master on commit b46c381:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/09/03/23_17_17_584/report/report.html?utm_source=github&utm_content=pr_comment

Details

20 Changed:

• spec/mdc-radio/classes/baseline.html

• spec/mdc-radio/mixins/mixins.html

• spec/mdc-switch/classes/baseline.html

• spec/mdc-switch/mixins/thumb-color.html

• spec/mdc-switch/mixins/track-color.html

[mdc-web-bot]

🤖 Beep boop!

Screenshot test report 🚦

20 screenshots changed from master on commit f417586:

• https://storage.googleapis.com/mdc-web-screenshot-tests/travis/2018/09/04/00_02_27_456/report/report.html?utm_source=github&utm_content=pr_comment

Details

20 Changed:

• spec/mdc-radio/classes/baseline.html

• spec/mdc-radio/mixins/mixins.html

• spec/mdc-switch/classes/baseline.html

• spec/mdc-switch/mixins/thumb-color.html

• spec/mdc-switch/mixins/track-color.html