Assemble the output.mako file ourselves instead of using Docassemble

[nonprofittechy]

I hate the extra work this creates for us, but it would let us do it in "pure" python for automated tests and for API-driven interview authoring in bulk

[nonprofittechy]

Basically we need to match the imports we pass to the mako.Template method with what Docassemble passes. We can keep the actual mako templates the same.

We don't need to worry about duplicating what Docassemble does with passing the interview context, as we should try hard to have most required information as an attribute of the interview object.