Add new iteration page behind feature flag

What does this MR do?

• Add form to create an iteration
• Create iteration with graphql request
• On success redirect to Iteration page (currently blank #218277 (closed))
• On failure, show flash message (improved in #219096)

Since this MR doesn't add the link to sidebar (that happens in !31335 (merged)), can navigate manually to -/iterations/new for a group. e.g. http://127.0.0.1:3000/groups/gitlab-org/-/iterations/new

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Ref #216156 (closed)