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/axe.ts

@@ -59,6 +59,10 @@ Cypress.Commands.overwrite(
                     "color-contrast": {
                         enabled: false,
                     },
+                    // link-in-text-block also complains due to known contrast issues
+                    "link-in-text-block": {
+                        enabled: false,
+                    },
                     ...options.rules,
                 },
             },