Allow users to collapse and expand Explore 'sections' (currently: Beta and Archived).

Mockup:

Details:

• The top section/main tiles don't need to be collapsible
• Important: The Beta section should default to expanded and the Archived section should default to collapsed.
• We probably want to remember the state with cookies (or whatever), but I don't think it's critical.
• Once this has been implemented we can remove the 0.5 opacity that is currently set on the Archived section/tiles.

Background:

RustyNova mentioned on ChatBrainz that the 0.5 opacity made it look like the archived tiles weren't clickable at all, this would alleviate confusion while still discouraging 'casual' users from visiting outdated or broken pages.