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

src/core/components/layout-utils.jsx

@@ -1,6 +1,5 @@
 import React from "react"
 import PropTypes from "prop-types"
-import { Collapse as OriCollapse } from "react-collapse"
 
 function xclass(...args) {
   return args.filter(a => !!a).join(" ").trim()
@@ -243,11 +242,9 @@ export class Collapse extends React.Component {
 
     children = isOpened ? children : null
     return (
-      <OriCollapse isOpened={isOpened}>
-        <NoMargin>
-          {children}
-        </NoMargin>
-      </OriCollapse>
+      <NoMargin>
+        {children}
+      </NoMargin>
     )
   }