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.
Somehow the `must beSome` seems clearer to me but that's likely a matter of taste