add support for embedding resources in the markdown editor

[alicewriteswrongs]

As a site content author, I'd like to be able to embed course site resources into course site markdown content (pages and whatnot).

Designs and Mockups

We have discussed using the React in a widget example as something of an MVP UI that we can then build off of. This also provides most of what we need technically to get React running inside the editor, display things around it which have editor callbacks, etc.

Acceptance Criteria:

• add support for 'SiteContentEmbed' block in CKEditor #452
• add UI for fetching resources, inserting them into CKEditor #453
• add rich UI to display inserted resources in CKEditor #454
• Resource picker UI #500
• add support for linking to resources to the Markdown editor #536

[alicewriteswrongs]

looks like this can be closed