If a label has been manually removed, fluttergithubbot should not re-add it

[goderbauer]

When somebody decides that a label is not relevant for a PR and removes it, the bot should not re-add it when a new commit is pushed.

/cc @dnfield

[goderbauer]

Come to think of it, maybe the bot should only add labels when the PR is first created. Additional pushes should not change the labels because that often causes the PR to fall of the radar for a certain team:

Say, I review a framework PR and it's determined that this change requires a change to a material test, the next guy the author pushes the PR, it's tagged "material" and no longer appears in the framework PR queue.

[goderbauer]

Example PR: #48900

It's not really relevant for "material" or "cupertino". However, those labels are re-added every time I push.

[dnfield]

It would probably be easier to just have it not add labels on subsequent pushes - but then that means if new files are touched that would be relevant to those teams, they won't automatically get added.

[godofredoc]

@keyonghan @ricardoamador would you mind taking a look?

[ricardoamador]

@godofredo I can take a look.

[ricardoamador]

This isn't happening via autosubmit since it only ever removes the labels, this is happening in cocoon but admittedly this would be easier to fix if I knew which label caused this bug to be open. If it is labels in general then that can be fixed but this is not related to autosubmit.

[matanlurey]

Was replaced by https://github.com/flutter/flutter/blob/master/.github/labeler.yml.