Use owners instead of authors in the UI

[emgarten]

If the owners field exists in search or registration metadata it should be displayed instead of the authors field. The owner information is official and can be trusted from nuget.org. Authors on the other hand is just set in the nuspec and could be misleading.

Local package sources will not contain owner information, those may continue to display the author information from the nuspec.

[yishaigalatzer]

#442