Refactor textanswers access control

[jooooosef]

Taken from #2232 (comment), which also includes some pointers on a possible implementation approach.

In #2232, we needed information about whether a user would be allowed to see answers by categories (e.g. "general textanswers", "your own textanswers", ...) to dynamically enable buttons that show answers of those categories on a results page.

Our current framework for answer access permissions isn't able to answer those questions, as a workaround, we duplicated access control logic in the evaluation_detail view. This is suboptimal as it might get out of sync with the actual permissions.

It would be nice if we could refactor our framework to "think" in terms of those categories when answering the question "can user X see answer Y", so that we are then able to answer the "can user X see answers of category Y" as well.