Implement autoscaling of expedition's rewards

[mdziekon]

Request background

In #123 the new and rewritten expeditions system has been merged into master. However, it's "reward system" is currently set to a flat reward value. This value (or values for any new events to be added in #124) should automatically scale based on these criteria:

• Ships in the fleet (more ships / more expensive ships should yield better rewards; however, there should be an upper limit on that)
• Server's speeds (resources extraction & overall game speed)
• Server's owner defined multiplier (additional variable / function for fine tuning on demand)

Related issues or PRs:

• Reinstate expedition fleet mission's code #90
• Re-enable expeditions (basic functionality) #123