REST API calls serving form fields are not using brief mode

[jeremystretch]

NetBox version

v3.0-beta1

Python version

3.7

Steps to Reproduce

1. Open the browser's developer console to inspect HTTP requests
2. Navigate to an interface and click the "connect" button
3. When the next page loads, notice that all the REST API calls triggered by form fields do not append ?brief=True to the API URL

Expected Behavior

All form-triggered REST API calls should employ brief mode.

Observed Behavior

Brief mode is not being used.

IIRC we identified this issue previously and switched to brief mode, bud had temporarily reverted back to full mode because of #6773 (which has since been resolved). Edit: There's a TODO note in api.ts referencing missing _depth attributes in some nested serializers, but as far as I can tell they're all present.