Remove overflow rules from .content

[Scotchester]

They were cutting off focus outlines on the form fields page.

They seem to serve no purpose anymore, anyway. Comments on the rules suggest that they were needed for the sticky footer, but removing them shows no ill effects on said sticky footer.

Fixes #529

Removals

• overflow rules from .content

Testing

1. Pull branch
2. npm start
3. Verify that the fields identified in #529 aren't having their focus outline cut off
4. Verify that the footer is still sticky on a short page, e.g., "Brand guidelines".

Preview this PR without the whitespace changes

Checklist

• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows the standards laid out in the front end playbook
• [x] Passes all existing automated tests
• [x] New functions include new tests
• [x] New functions are documented (with a description, list of inputs, and expected output)
• [x] Placeholder code is flagged
• [x] Visually tested in supported browsers and devices
• [x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)