Local missions should list distances in AU rather than Ly

[impaktor]

When listing table of all player's accepted missions, local deliveries all have distance "0 ly". Switch to AU instead.

Fixes #5755

Demonstration

Pondering

• do we have AU units defined elsewhere that I should use, or defining it in the local file is good?
• under which circumstances would mission.location:GetSystemBody() return nil, and what should one display in that circumstance?

[sturnclaw]

• Before accessing the result of SystemPath:GetSystemBody() you should check to make sure mission.location:IsBodyPath() is true. The location might be a path to a system rather than a body in that system, in which case the system body will be nil and dereferencing .body will raise an error.
  (Checking :IsSameSystem() means that the system body will have a "physical" Body representation since the system it refers to is currently "reified", and thus you don't need to worry about nil-checking .body.)
• Use ui.Format.Distance to display any in-system distances (passing the raw distance to the target). It will handle dividing by meters/km/AU as needed and format the result accordingly.
• Off the top of my head, we don't have a pre-built formatter for inter-system distances, so keeping the old string.format method for LY is fine.

[sturnclaw]

Looks good overall, if it's been tested then it gets my thumbs up. There's one variable name that should be changed before merge, but otherwise looks solid.

[sturnclaw]

This body is not a SystemBody, it's just a regular Body. Would strongly recommend the variable not be named systemBody as that usually implies the type of the variable is actually the SystemBody returned from GetSystemBody().

(Variable names are important in dynamically-typed languages...)