Fix milestone total weight is missing on the milestone page
What does this MR do?
Add the milestone total weight to the summary on the milestone page that was removed by this commit https://gitlab.com/gitlab-org/gitlab-ee/commit/6f3142465f901a68a2cc394e635121a27acd90d7.
Screenshots (if relevant)
- Before
- After
What are the relevant issue numbers?
Fixes #1340 (closed)