Add eslint-plugin-jsx-a11y (#1651)

* Add eslint-plugin-jsx-a11y.

* Fix npm script.

* Adjust npm scripts so test also runs lint.

* Fix existing lint errors.

* Don't break on a11y issues.

* Add role and tabIndex.

* Add vim and Mac files to .gitignore and .dockerignore.

* Handle htmlFor (partially), a that's actually a button.

* Fix missing tabIndex.

* Add cursor:pointer to load-more

* Revert change to load_more.

* Fixes based on review.

* Update yarn.lock.

* Don't try to install fsevents on Linux (hides warning noise).

Dockerfile

@@ -28,7 +28,7 @@ RUN BUILD_DEPS=" \
     imagemagick \
  && npm install -g npm@3 && npm install -g yarn \
  && bundle install --deployment --without test development \
- && yarn \
+ && yarn --ignore-optional \
  && yarn cache clean \
  && npm -g cache clean \
  && apk del $BUILD_DEPS \