Dual range test tag

[xhoenner]

Vemco now makes dual sensor tags which contain two range test sensors emitting at different battery outputs (to test long vs short distance receiver detections). When adding the second ID as a sensor during the tag registration process, the transmitter type options currently do not allow for another sensor of the same type (e.g. Range test).

The tag creation and editing pages should be modified accordingly so that one doesn't have to add dual range test tags as range test/pinger, example here. This fix should only apply to range test tags, not to other sensor types (correct me if I'm wrong @fjaine).