USE-101: Style tweaks to SuggestionPanel and refactor of SuggestionPanel variables

[djanelle-mit]

Developer

When we introduced the interventions from TACOS into USE the styles carried over had a few bugs with the USE styling. We had also introduced duplicate color variables and effects mixins that needed to be rolled into the existing partials for those.

This work fixes those style bugs and refactors variables and mixins:

• Adjusted spacing inside and above the intervention
• Removed duplicate font styles inside intervention
• Refactored color variables into variables partial
• Refactored effects mixins into the effects partial, and refactored some of those mixins and the applications in the header and footer.

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.

[jazairi]

Looks great in the review app, and the code is much cleaner now. Nice work!