Short labels for number of unread items

On the aside flux, suggestion of showing only a number (e.g. "7")
instead of the longer (e.g. "7 unread")
Merge branch 'shortUnreadCategory' of https://github.com/Alkarex/FreshRSS into Alkarex-shortUnreadCategory
Update du thème flat-design

Un peu plus de padding pour les labels "non lus"