Review custom template capacity

[amandabee]

Politico and Exactera have asked about custom templates. We think that it is probably time to start implementing this feature.

[mitchelljkotler]

@amandabee will scope out requirements and use cases

[amandabee]

Sample use cases:

• Exactera will not be making requests public and it's unclear whether they're technically "non commercial" requestors. They should be defaulting to a template that doesn't promise to make requests public.
• Politico wants to explicitly include language in every request, asking to avoid fees b/c they're a newsroom. They want to use the API to file.
• California Reporting Project has three core templates that they want to use. These templates are complete, meaning that they include the exact description of the materials they want included in the request. This use case might be too specific but it does not feel totally unreasonable.

@morisy These are the use cases I was expecting to support. Did you foresee others?