* Configure webpack to poll for changes in development

Vagrant on Linux/macOS hosts shared files via NFS, which doens't
support inotify-based watching of files. This tweak makes webpack
check for changes every second, and rebuild if necessary. This
removes the need to restart Foreman every time a frontend file
changes. Note that rebuilding is still a relatively lengthy
process.

The polling frequency can be changed to taste.

* Only poll in Vagrant

This tests for the presence of the VAGRANT environment variable to
determine whether or not we're in Vagrant. It is set in .env.vagrant,
which is set up to be included in the Vagrantfile.