feat(textfields): add new MediaInput component

[jzempel]

• BREAKING CHANGE?

Description

Used to apply correct styling to media textfield layouts:

<FauxInput mediaLayout>
  <MediaFigure>
    <SearchIcon />
  </MediaFigure>
  <MediaInput aria-label="Media Input Example" placeholder="Example media input" />
  <MediaFigure>
    <SettingsIcon />
  </MediaFigure>
</FauxInput>;

Detail

Also contains styling fixes provided by @zendeskgarden/css-forms@6.1.3.

Checklist

• 👌 design updates are Garden Designer approved (add the
  designer as a reviewer)
• 💅 view component styling is based on a Garden CSS
  component
• 🌐 Styleguidist demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 💂‍♂️ includes new unit and snapshot tests
• 📒 any new files are included in the packages src/index.js export
• 📝 tested in Chrome, Firefox, Safari, Edge, and IE11

[coveralls]

Coverage increased (+0.01%) to 94.792% when pulling 7375be3 on jzempel/update-css-forms into c19c5ae on master.