add support for website collections to the WYSIWYG editor

[alicewriteswrongs]

Once we can edit and save website collections we want to be able to insert these lists into page content.

We'll do this by supporting inserting a collection into Markdown via a hugo shortcode.

Acceptance Criteria:

• add custom 'Website Collection' node type, including Markdown shortcode ({{ websiteCollection $UUID }} ?)
• add a rich preview to the WYSIWYG editor to show the editor what the rendered content will look like (roughly)
• add support for rendering these shortcodes to relevant hugo themes (possibly using a public collections API on Studio to fetch data for them)

[alicewriteswrongs]

if anyone else takes this on, the documentation I wrote here will hopefully be helpful!

https://github.com/mitodl/ocw-studio/blob/master/docs/ckeditor/plugin.md