mint UD instead of deposit + disable burn (!307)

* review txmn

* fix(#231): next_cert_issuable_on is not used (!308)

* fix(#231): next_cert_issuable_on is not used

cert_meta.next_issuable_on is computed on genesis without this variable

* apply clippy

* remove comment

(used at line 1236)

(cherry picked from commit a6e8ba19)

* document burn as disabled

* replace raw origin by runtime origin

* fix #284

* use mint instead of deposit