Provide django secret with environment variable

[sei-jmattson]

Actions before raising this issue

• I searched the existing issues and did not find anything similar.
• I read/searched the docs

Is your feature request related to a problem? Please describe.

CVAT should only generate an app secret if one isn't provided.

Describe the solution you'd like

Check env var before generating app secret.

Describe alternatives you've considered

PR to follow.

Additional context

#8002 related, but not at odds with this issue.