Clear flashes on non-admin redirection

[mitsuhiko]

If the admin flashes and redirects outside the admin panel the user will see flashed messages when entering the admin panel again. However it makes no sense to display flashed messages that are older than 10 seconds or so in the admin panel.

I would propose adding a timestamp to flashed messages and only display flashes if they are not older than 5 seconds.

Caveat: this change must be forward compatible. If a message is flashed and Zine is upgraded between flashing and displaying the application must not die with a 500 internal server error just because the format changed.