add news subject to url (reborn)

Second attempt on displaying the news title in its url (original https://github.com/opf/openproject/pull/426).

This PR changes the `news_controller` to not use the generic `model_object` functionality to find news items. By doing that we can cleanup the parameters before querying the database (converting the url parameter to an ID).

Original text:

I noticed that OpenProject has poor urls for its news. This PR appends the subject to the news link to make them more memorizable and to know what to expect before clicking.

It turns

https://www.openproject.org/news/20

into

https://www.openproject.org/news/20-openproject-now-has-a-twitter-account

Note: both urls work today already. The PR only changes the links rails generates when using its helper methods.