Get rid of gulp-webserver

[toolness]

So with us no longer prettifying our HTML as of #692, gulp-webserver's auto-injection of script tags for live reload sometimes fails and garbles the whole page.

Even then, when livereload is working, there's still the problem of unexpected live reloads occurring if build times for certain assets are long, which can break a developer's train of thought.

Aside from that, it also looks like gulp-webserver doesn't load our PDFs properly (#773). It also hasn't been updated in 4 months and is on the gulpjs blacklist.

Given all these problems, and the fact that we've already got an express-based static server that runs our tests in PhantomJS, I recommend we just move to using that instead.