Email in org invitations should be case insensitive (#4153)

Resolved https://github.com/openvinotoolkit/cvat/issues/4096

CHANGELOG.md

@@ -51,6 +51,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Original pdf file is deleted when using share (<https://github.com/openvinotoolkit/cvat/pull/3967>)
 - Order in an annotation file(<https://github.com/openvinotoolkit/cvat/pull/4087>)
 - Fixed task data upload progressbar (<https://github.com/openvinotoolkit/cvat/pull/4134>)
+- Email in org invitations is case sensitive (<https://github.com/openvinotoolkit/cvat/pull/4153>)
 
 ### Security
 - Updated ELK to 6.8.22 which uses log4j 2.17.0 (<https://github.com/openvinotoolkit/cvat/pull/4052>)

cvat/apps/organizations/serializers.py

-# Copyright (C) 2021 Intel Corporation
+# Copyright (C) 2021-2022 Intel Corporation
 #
 # SPDX-License-Identifier: MIT
 
@@ -77,7 +77,8 @@ class InvitationWriteSerializer(serializers.ModelSerializer):
         membership_data = validated_data.pop('membership')
         organization = validated_data.pop('organization')
         try:
-            user = get_user_model().objects.get(**membership_data['user'])
+            user = get_user_model().objects.get(
+                email__iexact=membership_data['user']['email'])
             del membership_data['user']
         except ObjectDoesNotExist:
             raise serializers.ValidationError(f'You cannot invite an user '