Battenberg should store metadata on what project template version a project is currently using

[shoelsch]

While Battenberg allows users to upgrade their project no matter which template version they are using, it's helpful for users to understand the changelog delta.

An easy way for the user to do that manually is understand what project template version they are currently using, but Battenberg doesn't currently help users store that information. Storing the version will also help users understand how outdated their project template is.

I would propose that we store a hidden file (e.g., .template-version) that stores the --checkout value when executing the battenberg install or battenberg upgrade commands.