LG-12369 add accessibility checks to dashboard

[Sgtpluck]

Relevant Ticket or Conversation:

https://cm-jira.usa.gov/browse/LG-12369

Description of Changes:

This PR:

• Updates our accessibility test library
• Adds accessibility Rspec matchers to have parity with the IdP
• Updates tests
• Updates inaccessible code

PR Checklist:

1. [ ] Have you linted and tested your code locally prior to submission?
2. [ ] Have you tagged the appropriate dev(s) for review?
3. [ ] Have you linked to any relevant tickets or conversations?

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