REFACTOR: Consistency, A11Y, and less text for composer toggles (PR #12627)

This does a few main things:

  • Removes “show/hide preview” text from the composer (the button in the lower right).

  • creates a consistent .btn-mini-toggle style for all the toggles in the composer (preview, minimize, full-screen)

  • adds aria-labels to all the toggles in the composer (preview, minimize, full-screen)

GitHub

:rocket: