This commit outsources the flash message processing to it's own interface.
Overall flash messages now can have multiple flash types and each type can
store multiple messages. For convenience there are now four flash types
by default, INFO, NOTICE, WARNING and ERROR.
There are two concrete implementations: one preserving the old behaviour of
flash messages expiring exactly after one page load, regardless of being
displayed or not; and the other where flash messages persist until explicitly
popped.
