Automated re-pinning of certain pins

[KevinMGranger]

For example, Kevin's how-to-format-code message and davidism's reminder-on-how-to-ask-and-when-to-answer.

[davidism]

The same RO can't pin a message more than once.

[kms70847]

Theoretical workaround: periodically making a new message with identical content to the old one, and pinning that.