Add screen_options support for Dashboard

The dashboards know reminds of the users settings (Tab position,
visibility, etc.)
repair button: fix duplicate contributions

fixes #253
improve contribution counting

only count each contributor once per episode, even he legitimately has
multiple contributions

issue #253
refactor: rename function
refactor: move social repair into social module
refactor: move contribution repair into its module
Merge pull request #484 from podlove/changeable_dashboard

Add screen_options support for Dashboard
Merge branch 'release-1-10-20' of github.com:podlove/podlove-publisher into release-1-10-20
update readme
episode box: separate data from display
replace legacy hooks with new filter
fix ugly UI hack
and finally: reorder episode form elements
display title in episode form
update readme
improve redirects

- add “active” checkbox
- migration: mark all current entries as active
- add sortability
fix tracking data export
remove useless redirect label
update WordPress "Tested up to" value
auto-generate episode file slug