housekeeping: bundle size reductions (#4713)

* set new bundlesize goal
* preserve `GeneratorFunction` instead of all function names
* use js-yaml fork that doesn't require esprima
* set HTML content directly, instead of using React-Markdown
* use remarkable for all Markdown rendering
* add babel-plugin-transform-react-remove-prop-types
* remove SplitPaneMode plugin
* remove react-collapse
* remove AST plugin, and yaml-js
* trim Markdown HTML string output before rendering
* disable obsolete function name preservation
* add `getComponent` to propTypes

.babelrc

@@ -7,6 +7,9 @@
   "plugins": [
     "transform-runtime",
     "babel-plugin-transform-es2015-constants",
+    ["transform-react-remove-prop-types", {
+      "additionalLibraries": ["react-immutable-proptypes"]
+    }],
     [
       "module-alias",
       [