Move alertserver to use ../webtools/webtools.mk.

Not really as large of a change as it looks, mostly a lot
of moving files around and de-duplicating code.

 - Moved res/html/{alerts,commits}.html to templates/ and make them
   golang templated files.
 - Moved common header parts found in res/html/{alerts,commits}.html
   and moved them into the new templates/header.html.
 - Removed the duplicated res/js/common.js and res/imp/login and moved
   to using the common versions found in ../res.
 - Cleaned up dependencies in bower.json and package.json.
 - All HTML Import dependencies are now listed in elements.html, which
   is vulcanized into res/vul/elements.html and imported on all pages
   via templates/header.html.

BUG=skia:

Review URL: https://codereview.chromium.org/808153003