Update URL when editing iteration

What does this MR do?

Add /edit to the URL when editing iteration

Not using vue-router yet, went with minimal change to get the parts in place and ensure edit_group_iteration_url worked correctly

Doesn't add edit to project iterations, as they inherit from groups and we want to remove the edit option

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content