[web/native/keyserver] Start using new url facts

Summary:
Part of ENG-5153
Depends on D9398

Now we can start using the new /webapp/ and /keyserver urls in other places.
This also brings back the option of testing the web app with "staging" keyserver because it sets the urlPrefix to the one specified in the url facts (passed as a variable in html) (more context in D9145).

Test Plan:

• check that web and native migrations work
• check that uploading a new image works
• test that navigating to https://localhost:3000/comm/chat/thread/256%7C83932/ redirects to /webapp/
• I run the keyserver docs according to the instructions in the nix_keyserver_deployment.md to check if both new web app and keyserver endpoints work:
  • The webapp successfully loaded under http://localhost:3000/webapp/
  • In the network inspector I could see a successful request to the keyserver (http://localhost:3000/keyserver/get_initial_redux_state)

Reviewers: inka, kamil, atul, ashoat

Reviewed By: kamil, ashoat

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9451