Until now, we've relied on babel's "es2015" preset. This will always

transpile code to pure ES5, even if the syntax has been supported for a

very long time in browsers. This change moves us over to the "env"

preset, which uses browserslist and compatibility tables to

optimistically produce the best syntax. As part of this, we'll be

codifying exactly which browser versions are supported.
The update to uglify-js exposed a couple places where ES6 syntax had

snuck into plain .js files. Those have been fixed.
This change corresponds to an equivalent change in djblets.

Did development and package builds.
Ran js-tests.
Smoke tested the Review Board UI.