Remove unused database objects

See merge request ftp-team/dak!80

Setup Alembic

See merge request ftp-team/dak!78

Set owner columns for sequences

See merge request ftp-team/dak!79

RFC: Convert some simple classes into declarative mappings

See merge request ftp-team/dak!77

dakweb: /sha256sum_in_archive/<sha256sum> endpoint

See merge request ftp-team/dak!76

This patch creates a new /sha256sum_in_archive/<sha256sum>
endpoint in dakweb. The endpoint allows for searching for
files with a marching sha256sum in the archive.

I have also taken the time to fix docstrings of file_in_archive
which wrongly indicated that the return type was a dictionary.

There is no need to create empty files for source-only uploads.

The problem from c51e71bb occured
again with a call to `ssh` in a different place.  This should make
sure we don't run into this problem again...

Fix dinstall stage file

See merge request ftp-team/dak!70