Avoid spurious mentions

There are two cases where we could avoid mentioning the user:

- the user authored the issue/PR we're considering
- the user has been already manually mentioned (in the first message, or before the bot saw all the issues, e.g. when it got throttled by Github API rate limits)

Start by tracing the JavaScript GitHub Action's label-application and mention logic. Check how issue or pull-request authors and existing manual mentions are represented, then verify that those users are not mentioned again while other subscribers still are.

Written by the indexing model from the issue text.