[CI] Refactor publish_draft_release.sh (#1289)

* Refactor publish_draft_release.sh

* Switch to using arrays for storing labelled changes
* Combine changes for Polkadot and Substrate
* Change sanitised_git_logs in `lib.sh` to drop `*`s at the start
* Only look for priorities of medium or above (presence of one or more
  C1 label is enforeced by check-labels anyway, saves us some API calls)

* Ensure priorities >C1-low aren't labelled B0-silent

polkadot/scripts/gitlab/lib.sh

@@ -9,9 +9,7 @@ sanitised_git_logs(){
   # Only find messages referencing a PR
   grep -E '\(#[0-9]+\)' |
   # Strip any asterisks
-  sed 's/^* //g' |
-  # And add them all back
-  sed 's/^/* /g'
+  sed 's/^* //g'
 }
 
 # Checks whether a tag on github has been verified