feat: Improved table of contents (#1223)

* feat: New table of contents

* fix: Hide TOC in edit mode

* feat: Highlight follows scroll position

* scroll tracking

* UI

* fix: Unrelated css fix with long doc titles

* Improve responsiveness

* feat: Add keyboard shortcut access to TOC

* fix: Headings should reflect content correctly when viewing old document revision

* flow

* fix: Persist TOC choice between sessions

shared/utils/slugify.js

@@ -0,0 +1,8 @@
+// @flow
+import slugify from 'slugify';
+
+// Slugify, escape, and remove periods from headings so that they are
+// compatible with url hashes AND dom selectors
+export default function safeSlugify(text: string) {
+  return `h-${escape(slugify(text, { lower: true }).replace('.', '-'))}`;
+}