This PR is a follow-up to #54206 to make it so we only respect the
feature flag overrides for Zed staff (or when running a development
build).

Previously just the UI portion of the feature flags was disabled for
non-staff, but manipulating the settings file by hand would still allow
setting overrides that would be respected.

The only other setting on the "Developer" page of the settings UI was
the performance profiler. This was previously only available for staff,
as the entire "Developer" page was limited to staff, but it seems
reasonable to allow non-staff to see this.

Release Notes:

- N/A