Support library assets when doing TinyMCE Editor image URL rewriting.

[ormsbee]

The code for the HTML editor currently transforms course image references in the OLX to asset keys. But this substitution was written for courses and does not work for content libraries.

For now, I've disabled the rewrites for libraries entirely with 234496b

The approach we use in courses is to convert the "/static" asset link to an AssetKey and then request that from the base URL of the LMS.

Content Library assets change with every version, because the asset URL is based on the ComponentVersion UUID. It's possible that we could reset the document_base_url with every save. It's also possible that we could create a different static asset endpoint that always shows the draft version for the purpose of showing it in the editor. One note on document_base_url: because our assets start with a "/" by OLX convention, it would have to be converted to strip that leading slash before document_base_url would work properly. (So "/static/deer.jpg" would have to become "static/deer.jpg")

Acceptance criteria:

• images appear inline in the editor, and always display the draft version of the image
• the OLX is still stored with the normal convention of a leading slash, like "/static/deer.jpg".