A11Y: Fix several minor issues (PR #14436)

Fixes several issues found through automated testing via Accessiblity Insights (a Chrome/Edge extension).

Here is a screenshot of the report for the homepage at meta.discourse.org:

This PR should fix all issues except the color contrast ones (vast majority of which are handled by the WCAG color schemes):

  • removes aria-level attribute on dropdowns (disallowed for the listbox role)
  • removes the aria-sort attribute when there is no custom sorting
  • fixes a typo for list headings attributes
  • adds option role to listbox default item (dropdown’s select name)
  • moves navigation role to parent of ul element, on anonymous requests this has the added benefit of including signup/login buttons in the navigation landmark

GitHub

Looks good. Test failure seems unrelated.