Attendance completion certificate

[Xpirix]

Fix for #1455

Feature changes

• Added a new column Certificate Type in the Certificate model and showed it in the course attendees' list.
• The default type of certificate shown in the course attendees' list is the course's certificate type. This is useful for existing courses that have already issued certificates.
• Pushing the Issue Certificate using credits will open a modal to choose the Certificate type of the attendee
• The chosen certificate type will show on the create certificate confirmation page
• Regenerating one or all certificate(s) will not change the certificate type
• To change a certificate type, the existing certificate needs to be revoked first.
• Updated the unit tests

attendee-certificate-type.mov

[timlinux]

LGTM