[MICROBA-1109]
* Remove the requirement for the `auto_certificate_generation` waffle switch to be enabled in order for a learner to be eligible for a certificate. This switch should control _when_ a certificate is generated, not _if_ a certificate will be generated for a learner.
* Move the `auto_certificate_generation_enabled` check to happen before we try to generate v2 certificates in signals.py
* Update unit tests
* Update documentation
