Fix broken layouts in old browsers that lack flexbox support.

Very old browsers (in particular, Firefox 16 and iOS 8.4 Safari) don't
support flexbox without prefixes, which breaks sidebar and comment
layouts. Extend the browserlist config so that autoprefixer generates
flexbox prefixes for these browsers.

package.json

@@ -22,6 +22,10 @@
     "eslint-plugin-ignore-erb": "^0.1.1",
     "webpack-dev-server": "^3.8.0"
   },
+  "browserslist": [
+    "defaults",
+    "since 2012"
+  ],
   "scripts": {
     "lint": "yarn run eslint --plugin eslint-plugin-ignore-erb --ext .js,.js.erb app/javascript/src/javascripts"
   }