* 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
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