Test mode improvements

Re-works the "test mode" feature to rely on environmental variables defined in a new .env.development file which is automatically consumed by Vite in dev mode.

Within that file, an optional VITE_USE_TEST_MODE flag can be set to true. Doing this will allow the application to work in front-end only mode for a simpler local development experience (fake job server, WMF Gerrit for patch lookup, fake user credentials, etc).

CORS must still be enabled for Gerrit patch lookup in local development when test mode is set to true.