Rename source 'Webcam' to 'USB camera'

[leoll2]

The term 'USB camera' is more generic to represent all the cameras with a USB interface (including webcams).

In scope of this ticket, rename "Webcam" and similar terms with "USB Camera" in the code and documentation.

Example: in the source creation endpoint, th payload {"source_type": "webcam", ...} should become {"source_type": "usb_camera", ...}

Note: USB cameras can be enumerated through the GET /devices/camera endpoint.

[jpggvilaca]

@warrkan let me know once the PR is created so we can make the UI changes together