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`

src/style/_layout.scss

@@ -626,6 +626,32 @@
     }
 }
 
+.highlight-code {
+  position: relative;
+
+  > .microlight {
+    overflow-y: auto;
+    max-height: 400px;
+  }
+}
+
+.download-contents {
+  position: absolute;
+  bottom: 10px;
+  right: 10px;
+  cursor: pointer;
+  background: #7d8293;
+  text-align: center;
+  padding: 5px;
+  border-radius: 4px;
+  font-family: 'Titillium Web', sans-serif;
+  font-weight: 600;
+  color: white;
+  font-size: 14px;
+  height: 30px;
+  width: 75px;
+}
+
 .scheme-container
 {
     margin: 0 0 20px 0;