Upgrades webpack and other JS dependencies. This was primarily motivated
by GitHub reporting a vulnerability in Webpack (which shouldn't actually
effect our use of Webpack) and wanting to clean up some tech debt.
Note that equivalent functionality for url-loader and file-loader is now
included in webpack itself.
# Checklist for submitter
If some of the following don't apply, delete the relevant line.
- [x] Manual QA for all new/changed functionality
Zach Wasserman
a14228dd11