Note that we don't create duplicate notifications. I made use of the behaviour
of XSS-middleware: It removes all css classes from `<a>` anchors. Because
notifications rely on a css class `mention` which gets removed in a later
middleware, this gives us a nice behaviour for re-notifications without creating
duplicates. The downside is that it creates dependencies between middlewares and
it's not that obvious at all.
cc @mattwr18 @ulfgebhardt @appinteractive @Tirokk
