Refactor fleet missions' code | Part 01 | Resources pillaging calculation

[mdziekon]

Summary:

Currently there are three places with resources pillaging code duplicated. All of them should use a shared function to calculate how much resources should be pillaged from a raided planet / moon.

Changelog:

• Extract resources pillaging function into its separate file
  • Rewrite it into something easier to understand
  • Fix the issue of uneven spreading in cases where left-over storage capacity of one low amount resource is incorrectly redistributed to other resources (and therefore, wasted even though the left-over was sufficient to allow pillage the entire pillagable amount of another resource)
  • Write tests
• Use the new pillaging function in attacks handling functions:
  • Regular attack
  • Group attack (ACS)
  • Moon destruction

Related issues or PRs:

• Refactor fleet missions' code #91