MDL-44112: Download certificates of issued badges feature

Feature that extends/improves badges by adding functionallity
that allows generating/exporting badge also as a PDF certificate.

Certificate can be assigned to individual badges. Certificates
can be common (each user can create custom ones) or "official",
approved by some institution. "Official" certificates can be
assigned only by admins and managers.

Signed-off-by: Gregor Anzelj <gregor.anzelj@gmail.com>