POC: Move the context switcher into a `<GlDisclosureDropdown />

Proposal

Move the Context switcher into a Gl Disclosure Dropdown component to help with the visual heirarchy. Let's create a POC that showcases this first and go from there

This will solve a few issues, such as:

• Remove the Help footer from appearing in the co... (#406275 - closed)
• Adjust the styling and interaction pattern of c... (#406269 - closed)

Mockup

Note: This design is not final