tracking-issue: add support for OPTIONAL LABEL
Created by: bobheadxi
This adds a new marker, <!-- OPTIONAL LABEL: my-label -->
that allows you to add labels on a tracking issue that do not need to be present on child issues for them to be considered part of this tracking issue. This is useful for making tracking issues easier to find without adding labels to every single issue within the tracking issue.
Closes https://github.com/sourcegraph/sourcegraph/issues/28408
Tested by updating fixtures and testdata, and then manually adding a label, okr/ci-downtime
to the test API response for https://github.com/sourcegraph/sourcegraph/issues/25768 . The test still passes, indicating that the lack of okr/ci-downtime
on child issues do not cause the tracking issue to be emptied as it does now
Most of the diff is fixtures, it's not actually that big of a change I promise