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.

This reverts commit d3c5756f7a.

Updates LJM to fix grey overlay with "user is having connectivity
issues" caused by remote track overwrite.

It doesn't seem to work properly.

That is only required if we'd want our backend to authenticate on behalf of our
users. If the app is to authenticate directly it's not needed.

It's impossible to avoid conflicts on a long lived PR like this. Sigh.

This is required due to the Babel update.