ci: add hacktoberfest-accepted to merged PRs when it was missed

[anurag629]

A merged PR counts on the leaderboard only when it has hacktoberfest-accepted. Today codercops/chatcops#165 and codercops/chatcops#166 were merged without it, so they didn't count until the label was added by hand.

This workflow runs when a PR is merged and adds the label when:

• the author isn't a maintainer or a bot, and the PR isn't labeled spam
• the label isn't there yet
• every issue the PR closes is assigned to the author (the same rule and the same lookup as the claim check)

Otherwise it leaves a notice on the run and changes nothing. It only runs at the merge, so removing the label afterwards sticks.

It uses pull_request_target so it can label fork PRs, with pull-requests: write and issues: read. It never checks out or runs the PR's code.

How I tested this

I ran the script against merged PRs with the label call replaced by a print. codercops/chatcops#165, #166 and #167, #139 and #181, and codercops/discord-bot#29 would get the label. codercops/ogcops#103 wouldn't, since its issue wasn't assigned to its author, and a PR without a linked issue only gets the notice.