Fix Bug 4010 - API method to register does not return authentification token

[g-kartik]

Motivation and context

fixes #4010

When email verification is turned off in the all-auth settings, the rest-register api does not return auth-token key. This pull request solves the issue by returning the key in this condition.
Changes:

• Modify get_response_data of RegisterViewEx in iam views to add a new response field 'key'
• The value of key will depend on the email verification setting of all-auth. If email verification is 'none', its value will be request user's auth token key else it will be 'None'.

How has this been tested?

Tests

• Created tests for 'rest_register' api located in cvat/iam/tests/test_rest_apis.py to verify its response, both in case of email verification being turned on and turned off.
• From the root directory ran python manage.py test cvat.apps.iam.tests to run the tests

Checklist

• I submit my changes into the develop branch
• I have added a description of my changes into CHANGELOG file
• [ ] I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• [ ] I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[bsekachev]

Hi, @g-kartik

Thank your for the contribution!
Please expect delay in review because of New Year coming, and have a nice New Year celebration!

[g-kartik]

Hi, @g-kartik

Thank your for the contribution!
Please expect delay in review because of New Year coming, and have a nice New Year celebration!

Okay, Thank you!

[nmanovic]

LGTM, thanks for the contribution!