fixes several layout/css-glitches in the previous administration area and in the styles

[Fraenkiman]

Fixes #201 Fixes #144 Fixes #140

• In the admin area of the style Flatmaas-Rev is no longer tried via stylesheet to include the image nib-logo.png.
• In the preview of the Leggero style "Posted by + time" is now displayed right-aligned - as in the blog.
• The heading for posts and pages has received its own CSS class "entry-title".
• In all three styles, both heading sizes are now displayed correctly; as in the blog, so in the preview.
• The publication date is truncated from h2 and now uses h6.
• Font size h4 can now also be conveniently set via BBCode button.
• Long single line code in leggero-v2 style is now wrapped when printing.

With best regards Frank