uses named routes for topics/boards

This fixes the ugly routes displayed in the forum (e.g. https://www.openproject.org/topics/896?board_id=6) and shortens the code on top.

Unfortunately, the link_to_if_authorized helper does rely on the hash form for urls. Not all urls could be changed because of that.
Add changelog entry
Add OpenProject::Notifications as publish/subscribe mechanism

This allows code to register for certain events, we might later use
this later e.g. for sending mails on specific events like WorkPackage
updates.
add entry for notifications to changelog
Attach event handler for async loaded elements
Fix cuke

Disable warning message about unsaved data that is don't needed here.
Fixes the failing cukes in the CI