Add intercept of config.json (#10908)

* Add intercept of `config.json`

To help make sure that we run our tests against a known config, rather than
accidentally making requests to `matrix.org`.

* Remove now-redundant stubs of matrix.org files

Now that we intercept config.json, all this stuff is redundant.

* Reinstate fixture which is actually used

Turns out this is used after all

* Add a `map_style_url`

* disable failing axe check

cypress/support/e2e.ts

@@ -20,6 +20,7 @@ import "@percy/cypress";
 import "cypress-real-events";
 import "@testing-library/cypress/add-commands";
 
+import "./config.json";
 import "./homeserver";
 import "./login";
 import "./labs";