Add instructions on how to stash changes to files and the database for testing and demoing purposes.

Still a draft because I still have to actually test this going through up/stash/down/up/restore cycle.

Tested it and adding sudo, it appears to work as expected.

I also added a helper script for rebuilding the theme on branch changes, but I did not really test this one yet as it needs a rebuild of the app image. Latest versions seems to work after rebuilding the image.