Have a way to provide easy fixture data examples for each template

[brendanheywood]

At the moment each data source can provide test data, but in most cases like custom fields, it is pretty generic and you really want a test which has realistic shaped data. You may also want more than one test.

• I'm thinking of a way to provide simpler test fixture data which would be a text area which a chunk of json
• If provided this would replace the test data provided by the data sources (ie not merged)
• More than one fixture could be provided
• Each fixture is shown as its own example, so you can test in a single place things like optional fields, or more complicated things like array sources
• this is shown inline in the template editing page, but possibly moved under the save buttons