helpers: replace webhelpers.flash with own implementation

webhelpers is dead.

One small function implements pretty much the same functionality, using the
same session key so tests still pass, but also very simple and without external
dependencies.

It could be implemented with a class and different methods for adding, getting
and clearing. But internally, it would probably have pretty much the same
helper function has here. So let's just avoid the unnecessary complexity and
keep it simple.