Serialization is being called multiple times

[eliserichards]

It looks like the NimbusReviewSerializer validation is being called multiple times on save:

serialization-error.mov

Steps to repro:

1. Create a draft experiment
2. Add all info needed to ensure no other review errors are thrown (fill out the overview and audience pages)
3. On the Branches page, select a feature
4. Fill in branch names and descriptions
5. Add a deeply nested feature configuration, e.g.:

{
  "sections-enabled": {
    "hello": 1
  }
}

6. Click save

Expected results:
The serialization (and FML validation) is called once and the correct error message is shown

Actual results:
The serialization is being pinged continuously, and we're seeing variation in the UI from the different error messages from the FML

┆Issue is synchronized with this Jira Task