Extend packager key addition/removal issue templates by main key holder status
.gitlab/issue_templates/Remove Packager Key.md: Add a subsection for main key holders that specifically tracks the status on the key signature revocation.
.gitlab/issue_templates/New Packager Key.md: Extend the Checks section by a subsection specifically for main key holders, that ensures all main key holders have validated and signed a new packager key (with the help of checkboxes). Clarify wording in the keyring maintainer related subsection.
Closes #11 (closed)