Add setting to disable user features when user login type is not plain (#29615)

## Changes
- Adds setting `EXTERNAL_USER_DISABLE_FEATURES` to disable any supported
user features when login type is not plain
- In general, this is necessary for SSO implementations to avoid
inconsistencies between the external account management and the linked
account
- Adds helper functions to encourage correct use

models/user/user.go

@@ -1232,3 +1232,21 @@ func GetOrderByName() string {
 	}
 	return "name"
 }
+
+// IsFeatureDisabledWithLoginType checks if a user feature is disabled, taking into account the login type of the
+// user if applicable
+func IsFeatureDisabledWithLoginType(user *User, feature string) bool {
+	// NOTE: in the long run it may be better to check the ExternalLoginUser table rather than user.LoginType
+	return (user != nil && user.LoginType > auth.Plain && setting.Admin.ExternalUserDisableFeatures.Contains(feature)) ||
+		setting.Admin.UserDisabledFeatures.Contains(feature)
+}
+
+// DisabledFeaturesWithLoginType returns the set of user features disabled, taking into account the login type
+// of the user if applicable
+func DisabledFeaturesWithLoginType(user *User) *container.Set[string] {
+	// NOTE: in the long run it may be better to check the ExternalLoginUser table rather than user.LoginType
+	if user != nil && user.LoginType > auth.Plain {
+		return &setting.Admin.ExternalUserDisableFeatures
+	}
+	return &setting.Admin.UserDisabledFeatures
+}