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>

tests/mock_persistence.go

@@ -66,7 +66,7 @@ func (db *MockDataStore) Property(context.Context) model.PropertyRepository {
 
 func (db *MockDataStore) User(context.Context) model.UserRepository {
 	if db.MockedUser == nil {
-		db.MockedUser = &mockedUserRepo{}
+		db.MockedUser = CreateMockUserRepo()
 	}
 	return db.MockedUser
 }