Remove activity (#29056)

* remove activity frontend

* remove the activity page code

* remove GET api/activity/ route + tests

* regen dox for activity routes

- Not for GET /api/activity/ which has been removed.

* nix more things only used for GET /api/activity/

* pull out more only-used-for-get-api/activity/ code

* force namespace to be loaded (?) to be found

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Co-authored-by: Aleksandr Lesnenko <alxnddr@gmail.com>
Co-authored-by: Bryan Maass <bryan.maass@gmail.com>
Co-authored-by: Aleksandr Lesnenko <alxnddr@users.noreply.github.com>