Links to shared content fail when the base URL is of another tenant

Moved from oaeproject/Hilary#951

If I want to reference a shared document in an OAE discussion or document then I have to put in a link to the URL of the document. Users from my own tenancy will then be able to follow the link and see the document. However, if the user is in another tenancy, they will be shown the Access Denied screen - even if the document has already been shared with them.

For example, the URL https://oae.gatech.edu/content/gatech/eJNUAMMw1e references a document that has been shared with me. But clicking this link gives me the access denied page.
@nicolaasmatthijs: A couple of these issues are, at least from the standpoint of the standard markdown syntax, "working as designed" and our current [markdown parser](https://github.com/evilstreak/markdown-js) doesn't have any option to override the standard behavior. (Most of the other issues are easy to fix with some styling.)

To address the non-standard items, we'll either have to find a different parser that supports them as options (not sure if one exists), or effectively, create our own markdown parser. The OAE parser wouldn't necessarily be much more than a few regular expressions wrapped around a module such as `markdown.js`, but presumably we'll want to have a common code base for both the front end and the email generation. Rather than copy-and-paste source code between 3akai-ux and Hilary, would it be worthwhile to create a REST endpoint for markdown parsing that both could access? Or is there another way to share code between the two?