OCSADV-154: Remove "brightness" property

Removed unused/terrible `brightness` property (an arbitrary string) from `ITarget` and associated editors in the OT. Added code to parse these values into a proper `Magnitude` table on import, when possible, and note the disposition of the old value in a per-observation note.
Ah thanks. I knew there was a matcher but I couldn't remember it and the specs doc makes me crazy. I'll change it.