v0.5.0 - Django 3 support

This release make wagtailmedia compatible with Wagtail 2.9 and Django 3.0

Changelog

• Added interactive demo (#82). Thanks to @thenewguy
• Added setting WAGTAILMEDIA_MEDIA_FORM_BASE to simplify customization of the media form (#83). Thanks to @thenewguy
• Fix Wagtail > 2.8 compatibility (reshuffled imports)
• Fix Django 3 compatibility issues (#88). Thanks to @lohmander