drop unused models (build, criterion, settings, user), code & assets
    
    The 4 mentioned DB models haven't been used in years. Drop them to make the
    codebase and schema easier to maintain. The related functions (from controllers
    etc) are dropped as well.
    
    This also removes:
    * `EnumSymbol` and related classes, because it's not used anywhere and we
      already use SQLAlchemy's provided Enum which seems to work well
    * `util/koji_interface.py`, no longer required
    * some outdated images
    * a few unused javascript files
    
    Note that there are still some unused util functions lying around, like
    `check_api_rights()` or `check_api_key()`, which I decided to keep in case they
    could still be useful in the future.
    
    Merges: https://pagure.io/fedora-qa/blockerbugs/pull-request/204