DEV: refactor setting object mixin to helper class (#30529)

This PR moves the logic from the setting-object mixin to a helper class. I've opted to maintain the interface of the previous classes (ThemeSettings / SiteSetting) that used the mixed-in methods through aliases so that we limit the amount of changes here (these are referenced in the string form and across classes/templates).

Another option we may consider in future if we want to optimize for performance is straight duplication which would trade off this overhead of aliasing/chaining calls through the helper for some duplicate code - only 2 models require these methods at the time of this PR.

app/assets/javascripts/admin/addon/lib/setting-object-helper.js

@@ -0,0 +1,78 @@
+import { dependentKeyCompat } from "@ember/object/compat";
+import { isPresent } from "@ember/utils";
+import { deepEqual } from "discourse-common/lib/object";
+import { i18n } from "discourse-i18n";
+
+export default class SettingObjectHelper {
+  constructor(settingObj) {
+    this.settingObj = settingObj;
+  }
+
+  @dependentKeyCompat
+  get overridden() {
+    let val = this.settingObj.get("value");
+    let defaultVal = this.settingObj.get("default");
+
+    if (val === null) {
+      val = "";
+    }
+    if (defaultVal === null) {
+      defaultVal = "";
+    }
+
+    return !deepEqual(val, defaultVal);
+  }
+
+  @dependentKeyCompat
+  get computedValueProperty() {
+    if (isPresent(this.settingObj.get("valueProperty"))) {
+      return this.settingObj.get("valueProperty");
+    }
+
+    if (isPresent(this.validValues.get("firstObject.value"))) {
+      return "value";
+    }
+    return null;
+  }
+
+  @dependentKeyCompat
+  get computedNameProperty() {
+    if (isPresent(this.settingObj.get("nameProperty"))) {
+      return this.settingObj.get("nameProperty");
+    }
+
+    if (isPresent(this.validValues.get("firstObject.name"))) {
+      return "name";
+    }
+    return null;
+  }
+
+  @dependentKeyCompat
+  get validValues() {
+    const originalValidValues = this.settingObj.get("valid_values");
+    const values = [];
+    const translateNames = this.settingObj.translate_names;
+
+    (originalValidValues || []).forEach((v) => {
+      if (v.name && v.name.length > 0 && translateNames) {
+        values.addObject({ name: i18n(v.name), value: v.value });
+      } else {
+        values.addObject(v);
+      }
+    });
+    return values;
+  }
+
+  @dependentKeyCompat
+  get allowsNone() {
+    if (this.settingObj.get("valid_values")?.includes("")) {
+      return "admin.settings.none";
+    }
+    return undefined;
+  }
+
+  @dependentKeyCompat
+  get anyValue() {
+    return this.settingObj.get("allow_any");
+  }
+}