Certificates expiry dates

[ywarnier]

Is your feature request related to a problem? Please describe.
Certificates are generated but it is currently not possible to set an expiry date and track the corresponding expiries.

Describe the solution you'd like
Certificates are available as a single page for each certificate.

• The certificate table should get an additional "expires_on" field, with the date of expiry
• The gradebook table should get an additional field "expires_after" (int) with the number of days before expiry
• The certificate edition page should contain an optional field to indicate the number of days (after emission) for expiry. This should be transformed into a date for the given certificate
• The certificate edition page should offer a new tag with the expiry date (so that it can be decided to show it on the certificate)
• The certificate page should show a warning when the certificate is expired, with an action to be taken to renew it
• A new page should show, at the course/session level, the list of certificates with an additional information on expired certificates (a red tag or something like that)
• A new admin page should allow admins to see all expired certificates for the whole platform, to be able to easily provide information to anyone querying the validity of a certificate

[Coursenligne]

In addition would it be possible to link the expiry date with the course: when the teacher updates the course He just have to tick a box to indicate that the new version makes the certificate out of date. Then "just" change the certificate accordingly to the new expected expiry date.