JuliaControl Bot

I thought we should have an issue discussing how we want the @JuliaControlBot to work.
Currently

• it compares a PR to the reference images in https://github.com/JuliaControl/ControlExamplePlots.jl and replies with changes, whenever an update to a PR is made, as long as the PR is made from a branch that belongs to JuliaControl.
• It also creates a branch with the updated plots under https://github.com/JuliaControlBot/ControlExamplePlots.jl/branches/
• The bot can be manually called by writing @JuliaControlBot test-plots, by one of the members of the JuliaControl organization.

Should we reduce the number of posts the bot makes in some way?
Should it delete old posts?
Should it refuse to run on PRs that are not up to date with master? Otherwise the bot might report false changes.
Should we automatically pull branches into ControlExamplePlots when a PR is pulled into master? Or maybe automatically run an update ControlExamplePlots whenever master changes?

Proposed changes:

• Bugfix: It seems like the bot recognizes some types of comments as changes and runs a new check and post
• Add link to the new branch on ControlExamplePlots in the PR reply (Fixed in https://github.com/JuliaControl/ControlExamplePlots.jl/commit/d8514148de1aa0493345097716cbf25d52911a3f)

Start by reviewing the @JuliaControlBot behavior described in this issue, including its PR replies, manual @JuliaControlBot test-plots command, and branches in ControlExamplePlots.jl. Examine the linked ControlExamplePlots repositories and the proposed questions about comment handling, stale branches, master updates, and outdated PRs. Done means an agreed and implemented bot workflow, including the listed comment-recognition bugfix and any selected posting or branch-update rules.