Improve insights project automation

Created by: felixfbecker

This improves our insights project automation in some big ways:

There is now a GitHub API to get the project items for a specific issue or pull request, which this is now using. This means we don't fetch the entire project and iterate through it anymore, but rather fetch issues first (e.g. for a milestone) and then the associated items. This should make it more performant and help with running out of API quota.

Additionally it introduces a new automated column "Proposed by", which gets filled automatically with whoever assigned the milestone to an issue (or who assigned the team label, if the former happened first). This should save us some time in board reviews, when we would otherwise got through the proposed issues and ask "who added this?"/"can whoever added this give some context?" (and sometimes we don't remember who added it...).

To make this possible, this also moves the automation that adds issues to the board itself into the same PowerShell script and removes the separate Bash GitHub action that was running just for adding to the board.

This means it's also trivial to add commands in this place to send Slack bot messages when someone proposes an issue for an iteration (i.e. assigns a milestone), which can then serve as a starting point for an easy quick discussion thread in Slack.