add news subject to url

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.