fix h3 style, move out from settings to apps CSS

The defined h3 style looked more prominent than our h2 style, so I adjusted it. Also removed the bold, which looks out of place with our new slimmer design, as well because the h2 aren’t bold.

Please review @owncloud-designers

(Note to self: Much like the h2 and section styles around it here which are backported to stable6 https://github.com/owncloud/core/pull/7915#issuecomment-39198135, this should be backported too.) – We’ll see after this one is merged.