Expanding upon #272, should fix #268.

- templates now work from the template folder
- added some clarification to the docs to explain it.
- refactored out a `global $app`

@mablae I didn't know about namespaced templates in twig. Seems like a
cool feature, but in this case we actually _want_ twig to fall back to
the other folders, if it can't find the template. :-)