Fix 500 error on group-level Milestones page

What does this MR do?

Fixes #301002 (closed) by updating the "X more releases" text to not be rendered as a link when the milestone is a group milestone.

We don't have a group-level Releases page, so there's not an obvious page to link to in this context. Because of this, this text is just rendered as plain text instead. This fixes the 500 errors described in #301002 (closed).

Screenshots

Before	After

In the "After" image, note the difference in the "1 more release" text. For project milestones, this text is linked to the project's Releases page. For group milestones, this text is not linked.

Related to #301002 (closed)