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.
We also had some old version dependencies for babel, uglify-js, and

less. These didn't have any major bugs, but there were some security

notices on things higher up in the dependency chain, and updating gets

us some nice things (like a working calc() in less that doesn't

require weird escaping).

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