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.
We should adjust the activity and comments test so this behavior is validated. It should contain the following things at a minimum:

- Links being converted
- Links to OAE tenants being converted to a relative URL
- Dashes being converted to bullet points
- An XSS vector in the comment being correctly escaped
- Line breaks corresponding to line breaks in original comment