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 React-based rewrite looks whether there's a room name (in the
window's location) in order to choose between WelcomePage and
Conference. But app.js expects Conference to be rendered before it
builds a room name if WelcomePage is disabled and there's no room name.
A quick and dirty workaround is to render Conference within WelcomePage
so that the rendered result closely resembles index.html before the
React-based rewrite.

Converts ConferenceUrl to a class and binds an instance to the global
APP variable, as requested by HTerezov.

We need to make sure that on the page reload all original parameters
used to load the conference are preserved. New modules helps to manage
different types of conference URLs like the one used for invites and
the one for reloading the page.

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

Use location.href instead of location.pathname to make the URL absolute

This reverts commit 3d5af92c7a.