[ACL] Guest users, unowned bags, and other ACL details

I'm combining these here because they all relate to a single feature.

• #63 - This one required me explaining that both the bag and the recipe needed to have at least one ACL entry, otherwise the bag ACL would inherit the recipe ACL's default allow policy. So I'm setting it to default deny all that way people can safely forget to set it properly. This was already the case with owned bags and recipes, but now it will also be for unowned.
• #20 - This one is a bug in how the anon default permissions are defined. I've removed them from the code for the moment and will be adding them back in 0.1.
• #54 - I'm not sure what to do with this. These wikis should really be loaded from TiddlyWiki, not stored in the database, because if the user updates TiddlyWiki we would want that to be reflected in the wiki. It should probably just export them as single-file wikis and store them in the cache.
• #47 - This will be a config setting in 0.1.

Review the referenced issues #63, #20, #54, and #47 together with the current ACL, anonymous-permission, wiki, and configuration behavior. The work is complete only when the agreed 0.1 behavior is defined and implemented for unowned bags, anonymous permissions, wiki loading or caching, and the configuration setting.