Update SettingsController.php to save Slack Settings

This goes with #10438 that I just submitted about Slack Settings not saving.

This adds the necessary code to actually save the Slack Settings,
As they are already validated by the SlackSettingsRequest, this seems like an easy and low-impact fix.

app/Http/Controllers/SettingsController.php

@@ -674,6 +674,10 @@ class SettingsController extends Controller
             return redirect()->to('admin')->with('error', trans('admin/settings/message.update.error'));
         }
 
+        $setting->slack_endpoint = $request->input('slack_endpoint');
+        $setting->slack_channel = $request->input('slack_channel');
+        $setting->slack_botname = $request->input('slack_botname');
+        
         if ($setting->save()) {
             return redirect()->route('settings.index')
                 ->with('success', trans('admin/settings/message.update.success'));