custom profile fields: Extract a separate various for whether enabled.

This will make it possible to do this on a per-organization basis.

zerver/context_processors.py

@@ -107,6 +107,7 @@ def zulip_default_context(request: HttpRequest) -> Dict[str, Any]:
         'privacy_policy': settings.PRIVACY_POLICY,
         'login_url': settings.HOME_NOT_LOGGED_IN,
         'only_sso': settings.ONLY_SSO,
+        'custom_profile_fields_enabled': settings.DEVELOPMENT,
         'external_host': settings.EXTERNAL_HOST,
         'external_uri_scheme': settings.EXTERNAL_URI_SCHEME,
         'realm_invite_required': realm_invite_required,