refactor vote service

[MartinBenediktBusch]

This PR implements the following changes:

• requires a group category id on the questions-to-group-categories table.
• minimises the duplication of queries. For example, queries forum question once instead of each time when the plurality score gets calculated.
• seperates validation logic into its own functions to make the code more testable and easier to read
• increases test coverage of the vote service
• implements consistent error handling