🔥🔥🔥

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.

Prepare for React 16.

JSDoc comments didn't follow the ESLint rule for properly formatted
sentences.

BTW, I'm not blind to the fact that PasswordRequiredPrompt and
RoomLockPrompt participated in a birthing of source code through
copy+paste. (If we do not copy+paste, we will not have to fix one and
the same source code such as comments  multiple times.)

Moves native dialogs to use dialog container. Implements native Dialog that uses react native Prompt.

Split language detectors to be web/native dependent. Take care of strings that contain html.