fix(nimbus): fix rollout features form incorrectly submitting

[moibra05]

Because

• Rollout feature changes should be editable before Save without being persisted by intermediate HTMX updates

This commit

• Prevents intermediate rollout feature form updates from saving
• Renders temporary feature value editors for newly selected rollout features

Fixes #16119

[RJAK11]

I tested the changes locally and it looks great to me!

One small thing: When I fill out the rollout experience description and then go back to the old UI's branch page, the description field there is left empty, which triggers a validation error. It looks like the new rollout experience description is stored separately from the field the old UI uses. Just wanted to confirm that separation was intentional since we might have to make additional changes later on to avoid that validation error.

[moibra05]

I tested the changes locally and it looks great to me!

One small thing: When I fill out the rollout experience description and then go back to the old UI's branch page, the description field there is left empty, which triggers a validation error. It looks like the new rollout experience description is stored separately from the field the old UI uses. Just wanted to confirm that separation was intentional since we might have to make additional changes later on to avoid that validation error.

if im not mistaken the rollout experience field is a new field entirely and i wasnt sure whether the better approach was to introduce it as a new field with a migration, or to reuse a similarly shaped existing field like takeaways_summary.

the main issue i saw with introducing a new field is that all non-rollout experiments would leave it blank since it doesnt apply to them. that said, i can see that the current approach may make it unclear where the rollout experience description is actually stored, especially when switching between the new and old UIs