Working repository for debexpo

Baptiste Beauplatrequested to merge
lyknode/debexpo:apt-use-with-mentors into live

Resolve all currently flagged bugs regarding debexpo repository. (Closes #50 (closed), #15 (closed))

A new database migration tool is used to perform safe upgrade through sqlalchemy: alembic.

Migration scripts and configuration can be found in contrib/database while its documentation lies in docs/db_migration.rst.

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