feat: improve TokenBlacklistSerializer return values

[juanbailon]

• Raise TokenBlacklistNotConfigured exception when token_blacklist app is not installed, returning an error message and a 501 Not Implemented status code.

  • Previously, it returned 200 OK with no message when the blacklist app was missing, which was misleading because it implied the token was successfully blacklisted when, in reality, no blacklisting occurred

• Return a success message ('Token blacklisted') when a token is successfully blacklisted.

• Update tests to reflect the new return behavior.

Tackles Issue #742

Tests name changes

In test_views.py

• test_it_should_return_nothing_if_everything_ok --> test_it_should_return_message_if_everything_ok

In test_serializers.py

• test_blacklist_app_not_installed_should_pass --> test_blacklist_app_not_installed_should_raise_token_blacklist_not_configured