Fix query to create iteration in cadence

What does this MR do?

Three loosely-related changes:

1. Fix creating an iteration within an iteration cadence.
2. Set durationInWeeks to 0, and disable the field when automatic is false on 'Create cadence` form
3. Show 'Add iteration' dropdown based on automatic prop, rather than durationInWeeks

Requires iteration_cadences feature flag

The query was previously missing the input field and would throw an error when submit was clicked

Testing:

1. enabled iteration_cadences feature flag
2. go to Cadences page for a group (Issues > Iterations)
3. create a cadence with Automatic unchecked
4. in cadences list, click ellipsis, Add iteration
5. Fill in fields and save
6. 🎉

OR, bin/spring rspec ee/spec/features/groups/iterations/user_creates_iteration_in_cadence_spec.rb