Add issue templates for key actions

.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.