Allow querying of related and closing merge requests related to an issue

What does this MR do and why?

Allows querying the information at /closed_by and /related_merge_requests API endpoint from the issues-API in ruby conditions.

Builds upon the MR at !195 (closed)

For our workflow we needed a very similar feature, where Issues with open MRs should be assigned certain labels.

Example of usage provided here: !195 (comment 493992792) still applies

Changes compared to previous MR: Rebased, rubocop applied, unittests added for the API requests

Let me know if this is what you had in mind re unit-testing @godfat-gitlab - completely new to ruby, so might not be up to snuff there.

Closes #53 (closed)