merge two definitions of item types
    
    As identified in D1119, we had two locations for definining allowed item
    types. This merges it into a single location. Type handling might change
    again soon, but for this moment this eliminates code duplication.
    
    A few notes:
    * I picked `check.ReportType` as the single location, because it shows
    up in documentation and our code and tasks use that.
    * The new single definition is a superset of both (therefore we now have
    more item types documented and easy to use in reporting than we had
    before)
    * There was a clash between `bodhi_update` and `bodhi_id`, I picked
    `bodhi_update` because that's what our current tasks use.
    * I renamed `yum_repository` to `dnf_repo`, because no task currently
    uses it, so it seems safe.
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1133