implement HTML with preserved newlines for app descriptions
After much discussion, it looks like there is some consensus around using "HTML with preserved newlines" as the standard description format for apps in F-Droid. This implements that in fdroidclient. It is a small change, which improves things immediately. So I think it should be included even if some people do not agree with the new format: it is a small change that can be easily undone.
#1000 closes #1114 (closed)