Use the 3rd party location lookup API to accept third-party locations
in their native form and look up the corresponding portal room for
that location.
Also give the network dropdown some placeholder text.
Fixes https://github.com/vector-im/vector-web/issues/2374
Accept 404 errors from getting the config and start MatrixChat with no config, make other errors display a simple error message to prevent a completely blank page if the config does fail to load.