Revert "removed all browser-specific gradients as the recently (in 42ff6730) introduced svg makes them unnecessary"

This reverts commit dbd59867.

the new template allows to use a favicon in media/wiki/favicon.ico so we
should support uploading it as well.

See
http://stackoverflow.com/questions/805107/how-to-create-multiline-strings
for info on them.

Instead of a dedicated function to toggle the TOC we now have a function
that allows to use this functionality everywhere. This will be used to
toggle the sidebar in the mobile view (in an upcoming patch).

Note, this required some changes to the CSS (to make it more generic).
The CSS is still located in the TOC sections but should probably be
moved into its own section instead.

We had a lot of options lumped together under "Advanced Settings". I
introduced two more categories and move a few options to other existing
categories.
I also reordered the appropriate entries in the source files to make it
easier to find stuff when extending the options.