Decouple available user actions from video resources

[mathebox]

Is your feature request related to a problem? Please describe.
Additional user actions (like managing downloads) are currently only available for video items. Because of this, we check the type of the item’s content (item.content as? Video) throughout the codebase. When introducing new user actions on other resources, we would have to add additional checks in each section of the code. This indicates an insufficient level of abstraction. Therefore, we should decouple the user actions from video resources.

Describe the solution you'd like
Move logic to an extension of the superclass Content. Each content resource (subclass) can provide the available user actions on its own. Maybe use a separate protocol for this.

Affected Component
Content, Video+UserActions, ...