.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.
.gitlab/issue_templates/New Main Key.md:
Add issue template for adding a new main key.
.gitlab/issue_templates/New Packager Key.md:
Add issue template for adding a new packager key.
.gitlab/issue_templates/Remove Main Key.md:
Add issue template for removing a main key.
.gitlab/issue_templates/Remove Packager Key.md:
Add issue template for removing a packager key.