Add basic iteration report view

What does this MR do?

Ref #218277 (closed)

Split up !33566 (closed)

Basic info only

• Does not include edit button/view
• Does not include issues list

behind group_iterations feature flag. To enable for the gitlab-org group on dev:

echo "Feature.enable(:group_iterations, Group.first)" | bin/spring rails c

Screenshots

With iteration	Without (manually edit URL)