E2E test suite

[Ivshti]

• integration testing plan; some e2e tests (e.g. player, settings, login/logout, libitem sync) will be nice
• rigorous library/context tests: logging in, logging out, logging in as a different user, lib sync, etc.
• rigorous player tests: playing updates libitem, getting to the end sets to next video, etc
• data sanitization tests: addons cannot insert arbitrary html
• environment: the JS side should (1) TRY to load the WASM and (2) TRY to sanity-check the environment; if it doesn't succeed, it should show an error to the user
  • in reality, we have two "horribly wrong" panic states: failed to load WASM and then failed to load Ctx
  • we should reproduce these panic states in the E2E tests (e.g. by disabling localStorage)
• Optimization: web version: CI to use a headless browser to measure load times