github/codereview: Add check into InitialiseRelease
github/codereview: Add check into InitialiseRelease

Check whether the review milestone exists already and do nothing in case
it already exists. This makes the thing more robust in case someone
created the milestone manually.

Change-Id: b622fd4970
Story-Id: unassigned

salsaflow/salsaflow,salsaflow/salsaflow