Better group support for issuable resources

For existing issuable resources (e.g. issues, snippets, MRs) we have pretty good set of libraries which implement functionality used by issuable resources - methods for working with labels, notes, discussions. So far all of these resources (except personal snippets) were project-scoped, so also all shared modules expect that an issuable resource is scoped to a project.

Epics are issuables too (as their behavior is similar to e.g. issues), but scoped to groups instead of projects. When working on epics task I did some minor refactoring of the shared libraries to add better group support to some of them, but there are still many places where issuable's project is expected. It would be nice to have even better non-project support in these libraries.