Replace Cheetah template with jinja2 (or add additional support for jinja2)

Rationale:
- buildout uses ${var} configuration syntax, same as Cheetah. Using jinja2, template output is {{ var }}, making less pain to develop templates
- Cheetah is not known as Jinja2

Replacing Cheetah would mean breaking backwards compatibility with ZopeSkel templates, but bright future. Another option would to be to additionally support Jinja2.

The issue names no files, tests, or entry points. Start by locating where Cheetah templates are rendered, then determine whether the project will replace Cheetah or add Jinja2 support; done requires an explicit decision and handling the stated ZopeSkel backward-compatibility impact.

Written by the indexing model from the issue text.