Add support for .vue files

Since we've decided to standardize on .vue for our Vue templates (https://gitlab.com/gitlab-org/gitlab-ce/issues/29451#note_25359439) here's the work that needs to be done to make this happen:

update Vue library to 2.2.x (related #29043 (closed))
add vue-loader and its peer-dependencies to our package.json
decouple our babel config from webpack.config.js into .babelrc so it can be referenced by vue-loader
- this will probably break our code coverage tooling, so that may need some work
add the vue-loader config to webpack.config.js
ensure eslint understands how to read .vue files (use eslint-plugin-html for this)
create a meta-issue to start the process of converting our existing Vue classes and components to .vue so we can eventually remove the template compiler from our production bundles.