Improve news to contain an excerpt etc.

Merged username-removed-124398 requested to merge NicoAlt/fdroid-website:improve-news into master

This is mostly a migration of the old news page currently located at https://f-droid.org/news-and-reviews/. The modified "posted on ... by ..." is now also used on posts' page.

I did not use page.excerpt because it produced inconsistent pieces of text. Instead I used page.content and truncated it to 50 words.

It looks like this:

Firefox_Screenshot_2017-04-13T11-19-20.218Z

Fixes #7 (closed).