Refactor --template code

[micahellison]

Fixes #1736 and #1737.

Re-organizes some of the template code from #1667. Moving template concerns out of controller.py where possible.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

[micahellison]

In an attempt to fix this, I removed the Path return values in favor of strings. I like that it's more consistent with the rest of the code base (though I do like pathlib in theory), however the tests are still failing.

It looks like the problem is that the current directory is needed for os.path.abspath to work, but it somehow doesn't exist while running unit tests on Linux. I'm not sure what's going on there.

[wren]

💯