Unbold

1. unbold labels and folders. Was high time for that – otherwise folders look like they have »unread« items in them
2. explicitly unbold text by default, otherwise might be bold 

Number 2 is actually one of the funniest issues I ever encountered. With current master, set the language to French. The headings and much text will be bold …

Please review @owncloud/designers  (We also need to backport af7634696dc259c52f19181169a6622a874986f4 then)