Test CLIENT_SECRET_HASHER migration

[n2ygk]

#1020 enables hashing of client secrets and #1074 adds the missed migration required by it.

1. Create cleartext client secrets in a version prior to that commit
2. Upgrade DOT to master
3. Set a CLIENT_SECRET_HASHER other than None.
4. Run migration 0006_alter_application_client_secret
5. Confirm that the old cleartext client secrets migrated correctly and still work.

[n2ygk]

Maybe use https://github.com/wemake-services/django-test-migrations

[n2ygk]

I tested forward and back migrations, cleartext secrets, mixed with hashed secrets, etc. using Postman.

However, I was also able to use the hashed value of a secret and it was accepted as if it were the cleartext.

[n2ygk]

Forward and backward migrations are fine.
Opened new issue #1077 for the bug found.