Feature request #1935: Add versioning to snippets model

I believe I already have a request in here somewhere for versioning wiki articles (as a safety mechanism).

This ticket is about adding versioning to the snippets model (to bring them closer to GitHub gists). This time it's not about safety, but about providing the ability for visitors to see how a snippet has changed over time. (I'm not so concerned about the "view over time" aspect for the wiki, as it's not a community-driven wiki and so the usual goals of policing abuse don't really apply here).

I don't want to go the GitHub route of turning everything into a Git repo, or at least I think I don't. (For example, searchability is much more straightforward when all your text is in the database, and performance should be better too without the forking overhead.) If I can make this sufficiently general I may be able to use it for other models without too much effort as well.