Automatically resize the branding box (and logo) with the sidebar

This makes it possible to use any branding and any sidebar size
transparently, making it easier to fluidify the layout later.

The current downside is that we fix the box as a square, which may
add some bottom margin to the image depending on its aspect ratio.
To fix this we would need to read the aspect ratio of the image and
it in '%' as the padding-top style attribute, but I think it is not
worth it.
CSS: Fix the one pixel gap above the sidebar.
CSS: Fix the top bar gradient to follow $PX_BRANDING_LARG

This way even if we change that value, the gradient stays fit.
CSS: Simplify the header bar to avoid absolute positioning.

Since we now rely on the natural flow instead of absolute positions for
the sidebar and main contents, we have the absolute certainty that there
won't be any gap even if the user forces a bigger font. Also changes
won't have to be reflected at other places anymore.

It causes no aesthetical change at all.
CSS: Fix the gradient of the header bar on mobile devices

Previously, when the width was small enough, the gradient would
collapse into a bi-coloured white/brown area with a hard transition.

The new gradient looks nicer while being close to the old look for
medium-sized viewports.
CSS: Fix linear-gradient syntax for non-vendor-prefixed version.
 CSS: Replace absolute font sizes with relative equivalents.

 Absolute fonts sizes are not changed as compared with the previous
 version of the stylesheets, but are expressed as relative values.

 This commit also fixes the missing underline on the #logout "link".