Move to djangocms-video and djangocms-file

[Xiphoseer]

We currently use the defunct cmsplugin-filer for videos, pictures and downloads, which has been deprecated in 2016; sadly taking until novermber 2019 to mention (in that repo) that there are individual replacements available in the form of djangocms-picture, djangocms-video, djangocms-file and djangocms-link. Some of those have found their way into this project some other way but most importantly all of our videos that we just uploaded are in the wrong system now.

It's likely that we can lay out a clear migration from the old plugin to the new, as the last migration for cmsplugin-filer and the first migration for djangocms-video are nearly identical.