Fill must be properly specified.

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.

The lobby feature adds the possibility to lock a meeting and only allow people in after virtually knocking and going through formal approval

* Accessibility: Make the native toolbox item communicate that it is a button.

* Accessibility: If an item is toggled, mark it as selected for accessibility

* Accessibility: Make the toolbox a toolbar for accessibility

* Accessibility: Mark the bottom sheet as a menu for accessibility

* Fix typo, AccessibilityRole, not AccessibleRole

* Statement fix

* Appease the linter

* Fix linting errors for real this time

- re-add thepand icon, shaped like a pill
- round top corners

This has the side effect of showing the bottom navbar when the toolbox is open,
which is a nice thing since back is accessible.

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

Avoid using a Modal since those create trouble with the view hierarchy.

🔥🔥🔥

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.

We no longer need it since Rect 16 takes care of passing the context around.
It's also deprecated: https://atlaskit.atlassian.com/packages/core/layer-manager

The package now requires using a ModalTransition component
to handle animations. The existing DialogContainer component
has been split into native and web implementations to support
this change.