babel does not modify existing builtins by default. That means
some newer methods, such as Array.prototype.includes, may not
be available unless babel-polyfill is used.

It's no longer needed for building since Node >= 6 already has the minimum
required ES6 syntax. In addition, drop it from app.js since we use Webpack with
the Babel loader to transpile ES5 to ES6.

The first step towards enabling Facbook's flow in the project. The flow
configuration is pristine as generated by react-native upgrade.

In preparation for and as another early step in rewriting the Web
version of jitsi-meet using React, use Haste resolver which is able to
distinguish among platform-independent files, Web-specific and
mobile-specific ones.

Additionally, (1) make sure that Babel is capable of understanding React
files and (2) introduce React as a dependency.

The purpose is to repeatedly take small steps towards our goal and merge
them before they get in conflict with the separate ongoing advancement
of the Web version of jitsi-meet.

React Native's module bundler (aka packager) has its default Babel
preset - react-native/babel-preset - which it uses in the absence of a
custom .babelrc. Unfortunately, the default may be tripped by the
presence of a .babelrc in dependencies. Additionally, if the default
does not get tripped, the npm install of lib-jitsi-meet as a dependency
may fall into a recursion in which Babel attempts to transpile
react-native/babel-preset. To reduce the risks of stumbling upon such
problems, move Babel's configuration inside the Webpack configuration
file.

Enable ESLint on jitsi-meet with the same configuration and the same
goals as in lib-jitsi-meet.