Address voice a11y ux on user search input

[jsandoval81]

Requirements List

• None

Description List

• Added a global visually-hidden CSS mixin class
• Created a hidden submit input on the user list search form, removed submit from tabindex

Testing List

• yarn test:unit:all should run without errors or warnings
• yarn serve should run without errors or warnings
• yarn build should run without errors or warnings
• Code review
• Manually navigate (via URL bar) to octp/Users
  • Replace /octp with /aslp or /coun if needed to accommodate personal permissions
  • Ensure the user search still submits via keyboard as expected
    • Depending on compact permissions, this user management feature may not be allowed, but for purposes of this PR, you can simply check that the network request was submitted
  • Ensure the new submit button is not visible and is not in the tab order
  • Ensure the VoiceOver experience is more clear

Closes #223

[jlkravitz]

• review the pull request to get oriented
  • read the description of the pull request, which should summarize the changes made
  • read through every task on the Scrum board that's encompassed by this pull request
  • read the description of the commits that comprise the pull request
• skim all new code, in the context of existing code, looking for problems (knowing that the vast majority of new code will be covered by tests)
• review all tests
  • methodically review all new tests for correctness, quality of naming
  • look at code coverage of tests
  • determine what code isn’t tested, review that rigorously
• review documentation to ensure that it matches changes
• provide comments on the pull request on GitHub, as necessary
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  • for comments that are more general, attach the comment to a random line in README.md (as opposed to commenting on the pull request itself), to be able to use GitHub's ability to thread discussions on those comments
• run a security audit of dependencies (e.g. npm audit and pip audit) to ensure that there are no vulnerabilities that will be deployed to production (as opposed to vulnerabilities that only have an impact on the development environment)

[jlkravitz]

lgtm! @isabeleliassen