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.

Use a dimensions detecting root component. The Dimensions module does not
measure the app's view size, but the Window, which may not be the same, for
example on iOS when PiP is used.

Also refactor the aspect ratio wrap component since it can be taken directly
from the store.

Last, remove the use of DimensionsDetector on LargeVideo and TileView since they
occupy the full-screen anyway.

Fixes PiP mode on iOS.

* ref(Filmstrip): Optimize resizes.

* fix(thumbnails): resize.

* fix(thumbnails): Issue with height 0, width 0.

* doc(Filmstrip): Improve JSDoc.

🔥🔥🔥

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.

For the most part the changes are taking the "static propTypes" declaration off
of components and declaring them as Flow types. Sometimes to support flow some
method signatures had to be added. There are some exceptions in which more had
to be done to tame the beast:
- AbstractVideoTrack: put in additional truthy checks for videoTrack.
- Video: add truthy checks for the _videoElement ref.
- shouldRenderVideoTrack function: Some component could pass null for the
  videoTrack argument and Flow wanted that called out explicitly.
- DisplayName: Add a truthy check for the input ref before acting on it.
- NumbersList: Move array checks inline for Flow to comprehend array methods
  could be called. Add type checks in the Object.entries loop as the value is
  assumed to be a mixed type by Flow.
- AbstractToolbarButton: add additional truthy check for passed in type.

It's detected based on a size threshold.

We started on the way to responsive UI and its design with aspect ratio
and keeping the filmstrip on the short side of the app's visible
rectangle.

Shortly, we're going to introduce reduced UI for Picture-in-Picture. And
that's where we'll need another dimensions-based detector akin to the
aspect ratio detector.

While the AspectRatioDetector, the up-and-coming ReducedUIDetector, and
their base DimensionsDetector are definitely separate abstractions and
implementations not mixed for the purposes of easy extensibility and
maintenance, the three of them are our building blocks on top of which
we'll build our responsive UI.