Send Slack post when proposing issues

Created by: felixfbecker

This extends the project automation by sending a Slack post to #code-insights-planning every time somebody adds an issue to an iteration as proposed. Besides making sure we don't miss when we propose issues, that Slack message can then serve as a thread starting point to discuss whether or not we should move the issue to Todo and to provide more context on why it is being proposed, if necessary.

Also changes the iteration stats to be posted every Friday instead of only every 2nd (because why not, now that there is a dedicated channel).

Also fixes some bugs in the script.