Fixed Django templates for email and user guide

[azhavoro]

Motivation and context

resolve #2391

How has this been tested?

Manually

Checklist

• I submit my changes into the develop branch
• I have added 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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage remained the same at 61.58% when pulling ca55dc0 on az/fix_templates into dabb153 on develop.