Adds loadable contexts and email view urls

[wesleykendall]

@Wilduck this provides the following:

• Email Templates can now define a context_loader path to a function if they wish to pass the context through a function before it is rendered. This provides the ability to fetch various objects that may not be able to be serialized in the email context
• An EmailView and url was added that renders emails as a web page.

Future additions

• Add permission checking so that only recipients of the emails can view the email web page
• Elegantly handle cases when the context loader fails

[swans-one]

This looks good to me!