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.
Merge pull request #1044 from opf/fix/forum_urls
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.
Merge pull request #1047 from opf/feature/notifications

Add OpenProject::Notifications as publish/subscribe mechanism
add entry for notifications to changelog