add time elapsed info for import job

Allow non-existent hentaifoundry import elements for stories

References Issue #31

The 'media' and 'tags' elements can be missing for stories and the
default import job will not fail now.

Randomize access_key during initial database setup

The daemon.access_key is now generated on database initialization
using:

https://docs.python.org/3/library/secrets.html#secrets.token_urlsafe

By default, the access_key is ready to use.

Removed the helpful text to avoid confusion.

Replaces a default secret with a generated one.