Refactor issuables links to be more general

What does this MR do?

We are adding relations between epics and it would be weird to have issues when it can be also epics now. In order to prevent review of a huge MR that includes refactoring I created this one with the refactoring part only.

It refactors issuable links services and controllers to be more general. Mostly it is about naming (issuables/objects insteas of issues.

Additionally EpicRelations concern was created. This concern contains code that will be shared by EpicLinks (Epic - epic relation) and EpicIssues (Epic - issue relation)

The final MR in WIP is https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8637 .

What are the relevant issue numbers?

#8333 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer