silence (or fix) a bunch of eslint warnings

js/App.js

js/Article.js

js/FeedTree.js

js/Headlines.js

js/PrefFeedTree.js

js/PrefFilterTree.js

js/PrefHelpers.js

js/PrefLabelTree.js

js/common.js

js/form/ComboButton.js

js/form/DropDownButton.js

js/form/Select.js

js/form/ValidationTextArea.js

js/tt-rss.js