Avoid triggering pr-branch-auto-delete when simply watching a PR

[fregante]

• Closes pr-branch-auto-delete also deletes branch if it was not merged by the user #5868

Test

Any PR:

• that you can merge
• on a repo without native branch auto-deletion

Test bugfix

1. Open the PR in two browser, one with RG, one without
2. Merge the PR in the browser without RG
3. The branch should not be deleted

Test feature

1. Merge the PR
2. The branch should be deleted

[fregante]

I thought this feature was affected because it relied on the old behavior, but not really.

However it would greatly benefit from being rewritten with the new observer instead of using additional listeners, also because we have two separate UI pieces, one of which needs to be applied multiple times.

Related:

• MAIN AJAX issue, how do we fix it? #5895
• Drop most custom event listeners #5992

[fregante]

Also this would help rewrite pr-branch-auto-delete:

• Add one-event promised version: await delegate(document, 'a', 'click') fregante/delegate-it#34