Add milestone issue/MR links to release block footer

What does this MR do?

Adds a footer to release blocks that links to issues and merge requests in the associated milestone(s) as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/62402:

Note: For this iteration, this MR assumes that a release can only be associated with a single milestone. This will change in the future, so this feature will require a follow-up MR once this association is updated to be many-to-many.

UX specs

Screenshots