WIP: Resolve "Ability to create an iteration"

What does this MR do?

Experiment using HAML form instead of Vue. Basically copy/pasted milestone template then find/replace milestone with iteration. There is some incorrect and broken parts because I got a bit confused on the rails side, but FE looks basically the same, and we have some kind of form post happening.

Pros:

• loads faster
• visual consistency with milestone form
• smaller code change
• almost works

Cons:

• does this violate GraphQL-first?
• Sprint model doesn't quite play nicely, but I think I did something wrong

Does this MR meet the acceptance criteria?

Conformity

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #216156 (closed)