Add global certificate template

[ywarnier]

In the same way we currently have global document templates, add a system to upload and edit global certificate templates.

[NicoDucou]

On the certificate edition page, we already have access to the document's template so, we could simply add a certificate template to those templates and use it this way for the moment until we adapt the structure to be able to identify which templates are for documents and which templates are for certificates.
A template could be usefull for both documents and certificates so may be the best way would be to show all the templates available and on top of the list of template have a filter to choose which kind of template to show.

https://11.chamilo.org/main/document/edit_document.php?cidReq=NICODU&id_session=0&gidReq=0&gradebook=1&origin=gradebook&id=6229&curdirpath=/certificates

[christianbeeznest]

This feature is now fully implemented and merged into master.

Certificate templates are supported via filetype=certificate.

You can create, edit, upload, and set a default certificate per course.

Tags like ((user_firstname)), ((course_title)), etc., are replaced dynamically.

Please test it and let me know if anything is missing.

[ywarnier]

So this issue was initially about global certificates (i.e. certificates we can use in more than one course).
Nicolas suggested using the global templates (https://testing26.beeznest.com/main/admin/settings.php?category=Templates) but the images there are broken, so that would have to be fixed.

For the rest of it: yes, the global templates can be used to design the local certificate, so all good on that part. For the images bug with templates, I think we already discussed it and left it for post-beta, so I'm closing this.