Consolidate Admin Settings controllers: merge Get/Update pairs

Merge GetCompanySettingsController + UpdateCompanySettingsController into
CompanySettingsController with show() and update() methods.

Merge GetUserSettingsController + UpdateUserSettingsController into
UserSettingsController with show() and update() methods.

Absorb GetCompanyMailConfigurationController into
CompanyMailConfigurationController as getDefaultConfig() method.

Removes 5 single-action controllers, down to 4 Settings controllers.

app/Http/Controllers/V1/Admin/Settings/UserSettingsController.php

@@ -0,0 +1,29 @@
+<?php
+
+namespace App\Http\Controllers\V1\Admin\Settings;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\GetSettingsRequest;
+use App\Http\Requests\UpdateSettingsRequest;
+use Illuminate\Http\JsonResponse;
+
+class UserSettingsController extends Controller
+{
+    public function show(GetSettingsRequest $request): JsonResponse
+    {
+        $user = $request->user();
+
+        return response()->json($user->getSettings((array) $request->settings));
+    }
+
+    public function update(UpdateSettingsRequest $request): JsonResponse
+    {
+        $user = $request->user();
+
+        $user->setSettings($request->settings);
+
+        return response()->json([
+            'success' => true,
+        ]);
+    }
+}