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.
To be clear, I can tell that TCC writes the value for every target (science or guide) configured in TCS. I don't know if TCS uses it. Shane tells me the TCC has received empty values for that parameter since 2010B, so apparently TCS is not using it.
If the value is not present in the sequence, TCC assigns an empty value to it.