Add mutation concern to resolve a single issuable

What does this MR do?

From discussion !25628 (comment 294297427)

In multiple places we're repeating the functionality of including Mutations::ResolvesProject and then using Resolvers::IssuesResolver to resolve a single issue or Resolvers::MergeRequestsResolver to resolve a single merge request. It seems logical to move this functionality to a new mutation concern ResolvesIssuable and replace existing code.

In a new iteration, this concern could be adapted to use for epics by replacing the parent_path and using Mutations::ResolvesGroup instead.

This new concern will be useful in !25628 (merged) as well.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done