Restrict apportionment to CSB only

[Lionqueen94]

• Restrict apportionment API endpoint to CSB election with status Completed
• Restrict apportionment API endpoint to Coordinator CSB only
• Restrict frontend apportionment pages as well
• Adjust integration tests:
  • 1 test for complete csb -> successful apportionment
  • 1 test for incomplete csb -> no apportionment
  • 1 test for gsb -> no apportionment