The demo distribution isn't reproducible

Building the demo distribution several times yields different archives. Known issues are:

• the embedded database contains build timestamps from saving the documents
• the embedded Solr index differs
• the embedded observation ID is randomly generated
• the attachment metadata includes the build time

We should check if we can change the included timestamps to the build time and remove the files we don't control (so remove the observation ID and the Solr index, they will be re-generated on first run).