User management improvements (#1101)

* Show more descriptive success messages for User actions

* Check username uniqueness when creating/updating User

* Adjust translations

* Add tests for `validateUsernameUnique()`

Co-authored-by: Deluan <deluan@navidrome.org>

ui/src/user/UserEdit.js

@@ -87,7 +87,9 @@ const UserEdit = (props) => {
           },
           { returnPromise: true }
         )
-        notify('ra.notification.updated', 'info', { smart_count: 1 })
+        notify('resources.user.notifications.updated', 'info', {
+          smart_count: 1,
+        })
         permissions === 'admin' ? redirect('/user') : refresh()
       } catch (error) {
         if (error.body.errors) {