Extend SLO missed heatmap with no assignees or milestone

Context

• gitlab-org/quality/engineering-analytics/team-tasks#307
  • Specifically, gitlab-org/quality/engineering-analytics/team-tasks#307 (comment 1559415605)

Goal

In issues like this: https://gitlab.com/gitlab-org/quality/triage-reports/-/issues/13750#heatmap-for-slomissed-bugs in the SLO missed section please add a new table showing:

Columns: Each severity

Rows:

• Bugs with no assignee
• Bugs with a milestone that is in the past, backlog, or empty

Each cell should be the count of bugs in that category with a link to an issue search to find them.

(I realize that some bugs could be represented in both categories - no assignee and a concern with the milestone...that is ok)