Introduce 1280px max width container with responsive behavior

[djanelle-mit]

Developer

To help modernize our layout, we want to have a 1280px max-width content container in USE UI.

This work implments that by introducing a new set of variables for the content width and padding for respective responsive sizes. Then, those variables are applied to override the max-width and padding values provided by the theme gem.

Accessibility

• ANDI or WAVE has been run in accordance to our guide.
• This PR contains no changes to the view layer.
• New issues flagged by ANDI or WAVE have been resolved.
• New issues flagged by ANDI or WAVE have been ticketed (link in the Pull Request details above).
• No new accessibility issues have been flagged.

New ENV

• All new ENV is documented in README.
• All new ENV has been added to Heroku Pipeline, Staging and Prod.
• ENV has not changed.

Approval beyond code review

• UXWS/stakeholder approval has been confirmed.
• UXWS/stakeholder review will be completed retroactively.
• UXWS/stakeholder review is not needed.

Additional context needed to review

E.g., if the PR includes updated dependencies and/or data
migration, or how to confirm the feature is working.

Code Reviewer

Code

• I have confirmed that the code works as intended.
• Any CodeClimate issues have been fixed or confirmed as
  added technical debt.

Documentation

• The commit message is clear and follows our guidelines
  (not just this pull request message).
• The documentation has been updated or is unnecessary.
• New dependencies are appropriate or there were no changes.

Testing

• There are appropriate tests covering any new functionality.
• No additional test coverage is required.

[coveralls]

Pull Request Test Coverage Report for Build 18415225128

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 98.03%

---

Files with Coverage Reduction	New Missed Lines	%
app/controllers/search_controller.rb	1	95.51%

Totals
Change from base Build 18411425670:	-0.1%
Covered Lines:	846
Relevant Lines:	863

---

💛 - Coveralls

[JPrevost]

This looks good to me in both USE and GEO contexts.