Add wagtailvideos from https://github.com/neon-jungle.

The version in PyPi is a fork of an older version and incompatible
Django 3.

Pick current latest commit from the master branch, because the last
fixes seem to be usefull and there is no release for it.

Update python requirements with

    (cd requirements;  make)

The virtual environment can be update to the specified versions with

    pip-sync requirements/dev.txt

The <p> element at this level are wrong, because the StreamField blocks
will add their own <p> elements and nested <p> elements are not legal.
This structural errors makes the fallback text in a <video> element
visible, even when the video is playable.

Remove <p> element.

Create migrations with

    ./manage.py makemigrations