Cookie popup improvements (#3976)

Fixes #3925.
- Cookie consent preference is now stored in local storage rather than
using cookies
- Improves on the existing cookie banner by adding a `Learn More` option
where users can read more about the essential & analytic cookies, and
toggle analytics specifically.
- If analytics are not used, the user is no longer given the option to
opt out, as Hypha only uses essential cookies by default.
- Added a link/button to the footer default that allows the user to
re-open the cookie prompt without having to clear it from their local
storage.