#80(wpum custom fields) adding repeaters as subfields of a repeater should be allowed

[reygcalantaol]

Resolves #https://github.com/WPUserManager/wpum-custom-fields/issues/80

Description

Testing Instructions

1. Edit fields (Users -> Custom Fields)
2. Customize fields
3. Add a repeater field
4. In the repeater field dialog, add another repeater field.
5. Save the field settings.
6. Go to user account page in the frontend
7. Click Settings tab, edit the fields within the repeater
8. Save the data.

Pre-review Checklist

• Issue and pull request titles are properly formatted.
• Acceptance criteria have been satisfied and marked in the related issue.
• Unit tests are included (if applicable).
• Self-review of code changes has been completed.
• Self-review of UX changes has been completed.
• Review has been requested from @polevaultweb.