Introduced a new hook 'SkinAfterContent' that allows extensions to add text

Introduced a new hook 'SkinAfterContent' that allows extensions to add text
after the page content and article metadata. Updated all skins and skin
templates to work with that hook.

The hook is added in the newly introduced Skin::hookAfterContent(). It
couldn't be implemented anywhere else as we want to be able using a single hook
in all skins:

Some skins are based on SkinTemplate (e.g. MonoBook), while
some directly extend the Skin class (like CologneBlue). The Skin based ones
collect their output from several functions from Skin class, while SkinTemplate
prepares an array with all the needed data and passes it to the skins using it.

Thus we had to create said new protected function for running that hook. SkinTemplate
pushes the function's output into the data array. The Skin class based skins
directly use the function's output.