enhance: scrollable + downloadable HighlightCode (#4397)

* Auto hidding content that is longer than 600 characters long.

* Added basic downloading

Slightly broken

* Better downloading

now downloads file on button click

* Fix the angry linter

* Fix dist

* Removed collapsing, added scrolling.

* Code clean up.

* CSS fix.

* prevent HighlightCode from scrolling entire document

* center "Download" text in button

* `this.downloadJSON` -> `this.downloadText`

we're always saving as `.txt`, so JSON is a misnomer

* hide Download button behind option `downloadable` prop

* `file-saver` -> `js-file-download`

package.json

@@ -52,6 +52,7 @@
     "getbase": "^2.8.2",
     "ieee754": "^1.1.8",
     "immutable": "^3.x.x",
+    "js-file-download": "^0.4.1",
     "js-yaml": "^3.5.5",
     "lodash": "4.17.5",
     "matcher": "^0.1.2",