tracking-issue: Keep track of changes to planned work
Created by: tsenart
This commit makes it so that issues that were originally planned in an
iteration (labelled with planned/$MILESTONE
) don't disappear from the
tracking issue, and are instead crossed-off.
This allows us to keep track of changes to planned work throughout the iteration.
A follow-up PR will update our documented process with an extra step of
labelling all issues in an approved tracking issue with the
planned/$MILESTONE
label.