Parallelize prod and dev builds. Cleanup webpack.
Parallelize prod and dev builds. Cleanup webpack.

Because CI always builds both dev and prod configurations now,
parallelize the tasks using multitask in Rake.

Also turn the webpack generation commands into first-class tasks and
make `rake serve` run the webpack watcher along with the jekyll watcher
so all code changes automatically build.

department-of-veterans-affairs/vets-website,department-of-veterans-affairs/vets-website