Routing: Discover Itemid in Router for com_newsfeeds

This change moves the code that discovers the right Itemid for this content item from the NewsfeedsHelperRoute class into the component router. This means that also URLs in the content get the right Itemid and we don't need to pre-calculate them during the time the item was created. It also means that all URLs are treated correctly and not just those going through NewsfeedsHelperRoute.

This makes NewsfeedsHelperRoute obsolete. The changes that I'm proposing might interfer with PR #5140. So #5140 should be merged first in order to test this one properly. That means that also #5264 and #5276 should be merged before this one.

This touches com_newsfeeds. Further components are done in other PRs. I also wanted to keep it testable, since no one could be expected to test all components at once. See the first PR for this in #5285.

This was made possible through the generous donation of the people mentioned in the following link via an Indiegogo campaign: http://joomlager.de/crowdfunding/5-contributors