feat(a11y): Globally improve accessibility for screen reader users

Upgrade eslint related packages
Fix new errors

Remove global variables from files
Change type to interface

Convert some files to TS
Refactor MuteEveryone and MuteEveryonesVideo dialogs. Move shared code to abstract components. Remove unnecessary code

Improve Dialog component to auto close on cancel or on submit
Change logic to determine whether a dialog is using the new component
Convert some files to TS

Use new TS config from @jitsi/eslint
Fix all lint errors

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.

Create a new component that uses Dialog. Reuse existing actions
for updating a participant's display name.