Server: multichannel support for displays

[opinsys]

User should be able to set multiple channels to be shown on a display. The contents of the channels should be shown after each other in the order the channels have been specified.

[lamikae]

The database schema and ActiveRecord model updates for display multichannel are in commit a51a40d0848c990a7f15f9e302ba418da2cb744d. The migration should update existing records.

The UI for adding new channels and changing their order is in f20da1135f8e339643a85e24c27d6e4031013dad.

Works with and without JavaScript, but with JS the user can use drag-and-drop to change channel order and dynamically add many channels without saving after adding each channel.

[lamikae]

Not yet backported into develop branch.

[ptoivola]

Also timing different channels, for example "evening channel" and "day time channel" on the same display is needed when different groups use the same building at different times.