Implement adding custom screen types / devices to Channel Manager

Two use cases:

As a developer, I need to add a custom device to a channel, with a custom screen width. This custom device should show up in the device preview dropdown in the Channel Manager. 

As a developer, I need to be able to set a default device for a channel, which is then preselected in the device preview dropdown in the Channel Manager when opening the channel.

The first use case is for previewing a wallboard (a large screen used in a shop, width a width ~5000px). We should verify that this works in the channel manager, and that the large device is not shown zoomed out to fit the screen but there is a vertical and horizontal scroll enabled instead.

See linked issues for more details.