Add BaseTemplateAfterPortlet hook to add html after portlets

Add BaseTemplateAfterPortlet hook to add html after portlets

Allows extensions to move away from using (or abusing) the
SkinTemplateOutputPageBeforeExec hook to add things in places
such as after the list of language links.

Also explicitly check that $template->data['language_urls'] is not false.
Extensions can set it to an empty array to have the language links
header be displayed.

Bug: 57094
Change-Id: I099fb2ff48e02a2e3564a484aea906263e40bb53