Omit making network requests with rpmbuild
    
    When building documentation, one of Sphinx extensions [1] transforms
    plain text references to issues to links to appropriate issue tracker.
    It also requests issue title and state (open/closed) from the tracker to
    decorate these links further.
    
    Single request is made for each issue reference in documentation. This
    can be very slow and unreliable.
    
    This patch disables generating such links when building RPM. Currently
    it only means that the documentation installed from RPM would contain
    plain text issue numbers instead of links. Online documentation will
    still contain links to issues.
    
    See also 911bb60fce98bfde19d3959f70e5be0a3c8a6b8d.
    
    [1] https://github.com/ignatenkobrain/sphinxcontrib-issuetracker
    
    Fixes #128