Only process .tmpl files as Jinja2 templates (fixes #132)

When deploying a zapp, we now only treat UI file ending in .tmpl as
Jinja2 templates. Other files are uploaded verbatim.

Testing with more than one UI file showed that we implictly depended
on the order of iteration over the set returned by _find_ui_uploads.
Sets are unordered, so the tests would fail. We now return a list
instead to get deterministic output.