Refactor permission checks with issues and merge requests project settings

Slight refinement for read/write permission checks for 04d44522 as discussed in !691. Currently it appears that there is [no role that only has read-only access to issues](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/permissions/permissions.md), but it could be possible.

/cc: @DouweM

See merge request !704