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.
The value has never had any structure; some values are things like "unknown" and "very bright!" ... so I rather doubt it ever had any real use. Maybe it makes it all the way through to the FITS headers or something?