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.
Right now TCC uses that value to configure one of the TCS target parameters. It is also used as a backup to configure star magnitudes in Canopus, but only if the right bands are not present in the magnitude list.
TCC could use the magnitude list and the observing wavelength to calculate the value, but it requires a change in the code. The bottom line is that this change will break TCC until it is modified.