Custom commit message templates for commits to VCS

This PR deals with committing changes and with the commits caused by initializing new files from templates.
The fallback message in python is supposed to be mostly failsafe and thus only replaces one variable. Most projects would probably still pick it over the current message, but the default message in `60-translation.conf` aims to improve messages even more.

For git folks: A first iteration of this included `Reviewed-by: %(user)s` lines for the default settings.
I figured to omit it after some discussion, projects can still easily add the line if they are interested.

Anything else that `commitdata` should hold?