Clarify that reporters can manage boards
The original issue (https://gitlab.com/gitlab-org/gitlab-ce/issues/17907) didn't specify this, but the code always allowed reporters to do everything here. I think that's correct because
- Developer permissions normally refer to code, and boards are to do with issues.
- Reporters can manage epics, as well as labels on issues, which are in a similar space.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60794.