Up until now we relied on implicit loading of middlewares and reducers, through
having imports in each feature's index.js.

This leads to many complex import cycles which result in (sometimes) hard to fix
bugs in addition to (often) breaking mobile because a web-only feature gets
imported on mobile too, thanks to the implicit loading.

This PR changes that to make the process explicit. Both middlewares and reducers
are imported in a single place, the app entrypoint. They have been divided into
3 categories: any, web and native, which represent each of the platforms
  respectively.

Ideally no feature should have an index.js exporting actions, action types and
components, but that's a larger ordeal, so this is just the first step in
getting there. In order to both set example and avoid large cycles the app
feature has been refactored to not have an idex.js itself.

Spot will need a way to submit call feedback using the iframe
api. For now expose a method on conference.js to submit that
feedback. Exposing on conference.js looks to be the existing
pattern... Also add an event to notify consumers of the iframe
api that feedback was submitted, as postMessage is async
and the notification can at least give some guarantee maybe.

I haven't updated documentation yet as I'm not confident
about this api.

* feat(feedback): convert to react and redux

- For styles, remove "aui-dialog2" nesting so existing styles
  can be reused.
- Remove Feedback.js and replace with calls to redux for state
  storing and accessing.
- Add dispatching to FeedbackButton instead of relying on jquery
  clicking handling so the button can be hooked into redux.

* address feedback

* remove calling to not show feedback for recorder and filmstrip

When callstats is not configured hide the button.

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.