Fix: minor CSS touch-ups for mobile support

[TrueBrain]

During testing for release, I found a few things that bugged me enough to fix it:

• Padding in header was 6px top, 10px bottom. This is fine for desktop, but on mobile it looked really weird. So changed that for both to 6px both.
• width: 100% causes images that are uploaded too big but are suppose to render smaller, to size badly. This would be a huge regression for all wiki's using TrueWiki. So I reverted that part. It will make big images boom on mobile, so we do need to find a solution there. This just isn't that ;)
• Changed the min-width from 992px, which feels random, to 1020px, the size of the header. This mostly removes an odd period where we do make the header smaller, before going to full mobile. This works for the default CSS, but any wiki making changes to the header is having a hard time with it. And those 30 pixels are not really going to make a difference in day-to-day use.
• The search-bar is relative, and as such uses height (even despite the object not being there). This creates a weird gap between header and main. Fix this by using a negative margin if non-mobile.

[TrueBrain]

@Qrbaker : I had to revert the width: 100% on images; I need to look at it better to find a solution that works for existing wikis (read: OpenTTD's) and still work on mobile. I created an issue for it :) (#142)