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/e2e/register/register.spec.ts

@@ -22,7 +22,6 @@ describe("Registration", () => {
     let homeserver: HomeserverInstance;
 
     beforeEach(() => {
-        cy.stubDefaultServer();
         cy.visit("/#/register");
         cy.startHomeserver("consent").then((data) => {
             homeserver = data;