feature: Help menu implementation

Closes #220, implementing the help menu in the top header bar.

Estimated size: small-medium, 20 minutes

Changes

Edits CSS variables to reflect the primary/secondary/tertiary styling defined in the the original UX Figma file.
Adds a SecondaryButton styled component. (purple outline that goes to full highlight color when hovered)
Adds outline to the modals and dropdowns and adjusts content styling.
- Adds the getContainer or getPopupContainer to all the places we are using dropdowns to ensure styling is applied.
Adds the HelpDropdown component, containing a menu of buttons and links.

Video walkthrough (🔊):