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 #29020 (closed).

UX specs

When no milestones are associated, no footer will be rendered.

When a single milestone is associated:

When multiple milestones are associated, the footer text will change to:

View Issues for milestones 12.3, 12.4, 12.5. View Merge Requests for milestones 12.3, 12.4, 12.5.

(From this discussion: !15764 (comment 213567676))

Screenshots