fix(filmstrip): show thumbnails in 1-on-1 with a fake participant

Filmstrip remote thumbnails display under certain conditions, as
defined in filmstrip/functions.web.js. Previously the raw
participant count was used, which included fake participants.
Using the selector getParticipantCount excludes fake participants,
causing YouTube thumbnails to remain hidden in a 1-on-1 call.

Remove some usages of deprecated lifecycle methods

Abstract the DialogWithTabs title so it can be reused with other comp…

- Derive the showOverlay state. When the sidebar should be hidden,
  the internal showOverlay state should remain true until the
  animation hides it. When the sidebar should show, the showOverlay
  state should become true immediately.
- Use PureComponent to prevent additional animation triggers
  instead of explicitly checking changes to the "show" prop.

Based on react-native docs, looks like animations should be
started after mount. Updating animation states I'm not certain
on so I moved it to componentDidUpdate and tested with the
live streaming label to ensure the component still animated fine.

To kill componentWillMount, call destroyLocalTrack after mount.
Navigation to the blank page was synthetically forced and no
UI issues were noticed, possibly because destroyLocalTrack may
already be async so destruction may already have been occurring
after mount.

Per react migration docs, initially fetching external data is
recommended to be done in componentDidMount.

updated node-sass version for node 10

This is required to get rid of a warning after react native update. See related commit in lib.

Update docs for AbstractRecordButton _mapStateToProps

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.

Fixes common problem where people following https://github.com/jitsi/jicofo#secure-domain have a syntax error, forgetting the comma after muc definition.

Adds max occupant module.

None of the breaking changes seemed to affect current
usage of react-i18next and light testing of features
and language switching did not produce issues.

This update is a pre-requisite for removing deprecated react
lifecycle methods, as older versions of react-i18next
have a higher order component that uses the deprecated
componentWillMount, and that issue has been fixed since 7.8.0.

* feat(welcome-page): Redesign.

* Style adjustments.