Improve source code linting for continuous integration (#5017)

* Add rubocop linting to docker:test rake task

* Add eslint JS listing to plugin files, and add SKIP_LINT variable

* Fix failing ‘polls’ plugin eslint

Using eslint-disable-line, as is done in core qunit tests such as `acceptance/group-logs-test.js.es6`

* Add plugin test eslint to travis config

* Merge some eslint lines

.travis.yml

@@ -47,6 +47,7 @@ before_install:
   - eslint --ext .es6 app/assets/javascripts
   - eslint --ext .es6 test/javascripts
   - eslint --ext .es6 plugins/**/assets/javascripts
+  - eslint --ext .es6 plugins/**/test/javascripts
   - eslint test/javascripts
   - rubocop --parallel