Refactor Related MRs into Vue by adapting Related Issues components

Problem to solve

In the context of https://gitlab.com/gitlab-org/gitlab-ce/issues/51862, we'll need to re-use Vue elements (assignees, (issue_)milestone).

Therefore, the HAML implementation of the Related MRs need to be repurposed into Vue, in order to achieve this without duplication of code.

Further details

This might involve generalising some components of the Related Issues and creating new ones, at least for Related MRs.

No UI changes will be made, only updating the code that implements this section to use Vue instead of HAML.

Backend will require an endpoint with the relevant information for the MR (given the GraphQL API is not GA yet)

Links / references