Support videos upload

[Tijani-Dia]

Add support for uploading videos.

[allcaps]

During a quick meeting with project contributors, we decided that this is the most wanted feature.

• Use Wagtail media to manage the video files.
• Slack and Telegram do a good enough job encoding the video. We can use the submitted video without further processing.
• Video processing (encoding, scaling, etc) is out of scope.