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.

React Native doesn't define __filename nor __dirname so do it artisanally. In
addition, this helps with centralizing the configuration passed to loggers.

🔥🔥🔥

Using anything non-serializable for action types is discouraged:
https://redux.js.org/faq/actions#actions

In fact, this is the Flow definition for dispatching actions:

declare export type DispatchAPI<A> = (action: A) => A;
declare export type Dispatch<A: { type: $Subtype<string> }> = DispatchAPI<A>;

Note how the `type` field is defined as a subtype of string, which Symbol isn’t.

Fragment now works on both React and React Native, so use it.

* Adds in memory log storage, to be used while testing.

Enabling it only when config.debug is set, a configuration provided by jitsi-meet-torture.

* Moves to using config.testing.testMode property for logs storage.

* Fixes comments.

Allows to bind a click handler to a TestHint.

When a mobile test wants to click an UI element it must be able to
locate it through the accessibility layer. Now the problem with that is
that there is currently no uniform way for finding element on both iOS
and Android. This problem is solved by TestHint component which takes
an id parameter which then can be specified in the corresponding java
TestHint class in jitsi-meet-torture to easily find it. By being able to
add a click handler to a TestHint, it's possible to duplicate original
handler under nested TestHint and then find it easily on the torture
side.

Adds the logic to render TestHint only when the test mode is enabled
in order to be able to put independent TestHints in other places than
the TestConnectionInfo component.