Link to custom domain docs

lib/constants.dart

@@ -35,6 +35,11 @@ const String kMacOSUrl = 'http://download.invoiceninja.com/macos';
 const String kLinuxUrl = 'http://download.invoiceninja.com/linux';
 
 const String kDocsUrl = 'https://invoiceninja.github.io/docs/getting-started/';
+const String kDocsCustomDomainUrl =
+    'https://invoiceninja.github.io/docs/hosted-custom-domain/';
+const String kDocsCustomFieldsUrl =
+    'https://invoiceninja.github.io/docs/custom-fields/';
+
 const String kForumUrl = 'https://forum.invoiceninja.com';
 const String kApiDocsURL =
     'https://app.swaggerhub.com/apis/invoiceninja/invoiceninja';

lib/ui/design/edit/design_edit.dart

@@ -462,8 +462,7 @@ class _DesignSettingsState extends State<DesignSettings> {
                 text: localization.viewDocs.toUpperCase(),
               ),
             ),
-            onPressed: () =>
-                launch('https://invoiceninja.github.io/docs/custom-fields/'),
+            onPressed: () => launch(kDocsCustomFieldsUrl),
           ),
         ),
         if (widget.draftMode)

lib/ui/settings/client_portal.dart

@@ -343,8 +343,7 @@ class _ClientPortalState extends State<ClientPortal>
                                 text: localization.viewDocs.toUpperCase(),
                               ),
                             ),
-                            onPressed: () => launch(
-                                'https://invoiceninja.github.io/docs/hosted-custom-domain/'),
+                            onPressed: () => launch(kDocsCustomDomainUrl),
                           )
                         else
                           SizedBox()