Add Sphinx django model directive

[Coperh]

• Sphinx role to print a formatted model field with its description and default value if it exists
• Sphinx directive that prints the same as above for but for multiple fields in the same model

[Coperh]

@swrichards @stevenbal I saw that you might be interested in this so I rebased it onto master and made a few small changes with the role format and the default string.