Also expose this in the native SDKs.

It's obsolete now, since config is handled in Redux. Also add a "defaultUrl"
prop so emdedding applications can select what the default base URL is.

The files react/index.native.js and react/index.web.js ended up having
very similar source code related to initializing the Redux store. Remove
the duplication.

Additionally, I always wanted the App React Component to be consumed
without the need to provide a Redux store to it.

As an intermediate step on the path to merging jitsi-meet and
jitsi-meet-react, import the whole source code of jitsi-meet-react as it
stands at
2f23d98424
i.e. the lastest master at the time of this import. No modifications are
applied to the imported source code in order to preserve a complete
snapshot of it in the repository of jitsi-meet and, thus, facilitate
comparison later on. Consequently, the source code of jitsi-meet and/or
jitsi-meet-react may not work. For example, jitsi-meet's jshint may be
unable to parse jitsi-meet-react's source code.