Add user-agent / device to video stats

[kontrollanten]

Describe the problem to be solved

Currently there's good stats about videos, but it would be interesting to group the viewers also by client type; mobile, tablet, browser and mobile app.

Describe the solution you would like

Store the user-agent for each viewer in order to be able to group them in the stats view. It would also be good to add a plugin filter to let plugins add custom grouping.