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.
There seem to be a few problems with the Markdown comments:

- A single line break seems to be removed when rendering. For example:

```
Line 1
Line 2
```

becomes

```
Line 1 Line 2
```

- The bullet point style should be a disc rather than a circle

![screen shot 2014-09-24 at 17 45 21](https://cloud.githubusercontent.com/assets/109850/4392005/6c9b92b8-440a-11e4-9ddc-5cd7bc423b56.png)

- HTML characters seem to be double escaped. They should just be displayed as they've been entered

![screen shot 2014-09-24 at 17 49 34](https://cloud.githubusercontent.com/assets/109850/4392044/d663a488-440a-11e4-8934-bcedb8d6dad2.png)

- Text within a single \` block overflows the end of the page. Text within a triple \` block is still shown on a single line, whilst it would be expected to be a block. I believe the Markdown content preview already has some rules to deal with all of these. We should consolidate these shared Markdown display rules into `oae.base.css` and create an `oae-markdown` class that will cause these shared rules to be applied.

- Blockquotes don't seem to be working

- Embedding images doesn't seem to be working