Implement WCAG keyboard inputs for AccessibleDropdown

[ShrimpCryptid]

Use Case

Extension off of #408 ("Align dropdowns with WCAG rules (keyboard inputs") and #428 ("refactor: Use react-select for selection dropdowns"). Implement WCAG/Aria spec in AccessibleDropdown, currently used for the Help menu.

Acceptance Criteria

• AccessibleDropdown should open only on click (not on hover) or spacebar button input.
• Up/down arrow keys should move the focused element in the list, and clicking again with the spacebar should select it and close the menu.
• Home/End should move to the top or bottom of the dropdown list.
• Styling/visual appearance should be identical to the original dropdowns.

Details

• Currently this is only used in Help dropdown.
• Look into WCAG rules for menus?
• Can we use react-select? Another component?