Support Markdown syntax for comments

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.
To be clear, a comment of the form

    This is a line
    
    This is a second line

will be parsed and converted into two separate HTML paragraphs:

    <p>This is a line</p>
    
    <p>This is a second line</p>

It won't be converted to a single paragraph with a double line break (i.e. two `<br>` tags). The test code above already verifies the expected behavior multiple times.