Feature/cleanup expired tokens

In order to keep the users engine independent of the tickets engine i added a generic load hook to the account model. The tickets engine then monkeypatches the account destruction and destroys all tickets before the user is destroyed.

The tickets are destroyed first so that even if things break there should never be tickets with an outdated user id.

I would have prefered to use super over using an alias_method_chain but I have not been able to figure out a way to make account a superclass of the account extension and still refer to Account from the users engine.

This is mostly for cleaning up after tests so far. But we might expand this to destroy all identities disabled before a certain date.

This way the identity model defines how identities should be disabled. We currently still destroy them. But it will be easy and nicely isolated to change this next.

We blacklist based on three things:
* blacklist in APP_CONFIG[:handle_blacklist]
* emails in RFC 2142
* usernames in /etc/passwd

The latter two can be allowed by explicitly whitelisting them in APP_CONFIG[:handle_whitelist].

We stick to blocking names that have been configured as both blacklisted and whitelisted - better be save than sorry.