Create todos for expiring SSH keys

The goal of this issue is to create todos for "ssh key expired" and, in a second step (MR) also a todo for "ssh key will expire soon". This second notification then should auto-resolve when that same SSH key finally expires.

With this work we will:

• bring todos a little bit closer to feature parity with email notifications.
• create a new type of todo. We can use this work as reference for future additions to todos.
• create the first type of todos that has no project/group relation. As part of this change, we will have to remove that validation on the todo model and make sure nothing breaks with that change.