Add competition-rules to database, with change history

[JoselleAstrid]

From the start, perhaps each game would just have a rules text-field and we'd copy it verbatim from the old FZC. There should be change history so that whenever the rules are edited, the changes get tracked - differences, date, and perhaps the person who edited (and ideally, this would be presented in a wiki-like format on the website frontend).

Don't forget the FZC-wide guidelines as well.

Ideas for later, which don't have to be accommodated in the DB structure immediately:

• A general rules text which applies to all games. Right now, the game-specific rules have some very broad clauses like "no cheating" and "only submit records done by yourself", which are repeated for all games. If stuff like this is still necessary to say, then they should be presented as general rules, not game-specific.
• It may or may not be cleaner to track individual rules (or rule sections) as individual DB objects, rather than the entire rule text of one game as one DB object. Not sure though.