Magazine Rules are not federated

[BentiGorlich]

Describe the bug
The rules of mbin magazines are not visible on remote instances, because they are not part of the activity pub representiation of a magazine

Expected behavior
Just append the rules to the bottom of the description property

[BentiGorlich]

Atm rules are simply ignored, so they do not get federated to another server. Solution: make the rules part of the summary with a simple "# Rules" header. We could then say: if the last header in a summary == "Rules" -> ignore the header and put the text below it in the rules field. That way we have the most compatible and functional behaviour

[BentiGorlich]

duplicate of #415