Mercury Editor Integration

Features present in this branch:
* Mercury Editor integration for WYSIWYG editing for documents
* Replaced edit links with Mercury Editor links
* Tests for happy/sad paths to Mercury Editor
* Updated documents show/index template to support safe html rendering
* Embedding image/videos in documents

What is **NOT** included:
* Image/Video uploading, it deserves a separate user story
* Advanced Mercury Editor features, such as *snippets*, *history* and *notes*