Rewrite the permissions system

[pjc09h]

This is a long neglected subsystem that more or less works, but it too fragile for my liking. The old system is wrapped in simple functions like can and cant, that are also available to Twig templates.

The database storage has also been upgraded to JSON as opposed to PHP serialization, providing a possible security benefit in the sense that I can't really trust the legacy codebase not to do something stupid.

It's time for clean and simple permissions and roles overall, e.g., edit own torrents and edit any torrents.