feature: video background

[fmiccolis]

Prerequisites checklist

• [x] I have read the contributing guidelines.

What is the purpose of this pull request? (put an "x" next to an item)

• [ ] Documentation update
• [ ] Bug fix
• [x] New feature
• [ ] Other, please explain:

What changes did you make? (Give an overview)

Starting from the issue #641 I've created this feature to use local uploaded videos or youtube video/livestream as background in the main page.

Is there anything you'd like reviewers to focus on?

I tried to keep the commits separate so that there is an opportunity to create different pull requests for individual features (new video setting type, ability to upload video from the uploader, and adding the new properties in “User interface” to handle the new feature) @benjaminkott

[andi34]

Great! Thanks a lot! One thing: The cheese image to upload and image select setting change should go into one separate PR independent of the video feature.

[andi34]

Sperated here https://github.com/PhotoboothProject/photobooth/pull/808