Fix the data model to make deleting a project straight forward
    
    We used to run into some error when deleting a project, such as:
      StaleDataError: DELETE statement on table 'projects_groups'
      expected to delete 1 row(s); Only 0 were matched.
    which just entirely blocked deleting the project.
    
    We were also seeing a warning:
      SAWarning: DELETE statement on table 'tags_issues_colored'
      expected to delete 1 row(s); 0 were matched.  Please set
      confirm_deleted_rows=False within the mapper configuration
      to prevent this warning. (table.description, expected,
      rows_matched)
    
    It is only when we found the warning in the SQLAlchemy documentation
    about basic relationship patterns:
    http://docs.sqlalchemy.org/en/rel_1_0/orm/basic_relationships.html#association-object
    and when we applied ``viewonly=True`` to our secondary relationships
    that we were able to get ride of these errors and have our model behave
    as desired.
    
    We are also adding unit-tests to prevent this from occurring again.