The Dropdown component should be considered the dropdown folder going
forward. This should be distinguished as a different component that
composes the Dropdown component
Since the template itself is rather simple it seems preferable to have
duplicate templates that describe the actual component (PageTools,
VariantsDropdown and LanguageButton)
MenuDropdown is renamed Dropdown and styles that don't correspond to the
Dropdown component are moved to the relevant components.
MenuContents mustache template (future component) is added to DRY up
several of the new templates.
Bug: T320927
Change-Id: I220d0a477b3f05b278bd2cec7dd601c7b3b17c6f
