Added Search & Screen Options for the Contributor list
Sort contributor tables
Added Contributors to "All Episodes"

The contributors are listed. However, filtering by the slug is not
available yet.
Moved Contributor Menus to Episodes
Removed foo
fix contributor screen options
allow to filter episode list by contributor
contributors: add link to list all episodes
Merge remote-tracking branch 'podlove/filter_contributor_list' into release-1-9
contribs: track contribution count
sort contrib list by number of contributions
migrate contributions
add expert setting: dont force downloads
remove chosen from episode license chooser

... because I can't get it to work properly