use manual trigger for PR-Agent review

Currently, the PR-Agent always reviews after every push to a MR.

Due to cost and general annoyance, we decided that we want to manually trigger the PR-Agent review when the developer deems it to be ready for review.

Even though the PR-Agent supports gitlab web hooks, we choose to use a CI stage that has to be triggered manually (Pipeline -> select PR-Agent stage -> click on run). This is simpler and does not require a second server that listens for the web hooks.

ACCs:

• Fix CI to never automatically run the PR-Agent review stage
• Change the improve command to create threads instead of a table
• Increase number of findings as we have significantly less reviews now