Update issue templates

.gitlab/issue_templates/*: Replace allan with grazzolini when assigning tickets or when addressing main key holders. Streamline the checkbox system, by relying on less interaction from the main key holders side if possible (e.g. checks on new keys are done automatically in a merge request, so have contributors open the merge request). Add more documentation on what needs to be edited and how to provide data exactly (e.g. keyid format, clearsigned document).