Include entidi.com features in plain silverstrap

Added some quite common features to bare silverstrap:

* improved table of contents (#ss-toc) with affix and scrollspy;
* enable Bootstrap gallery if found (#ss-gallery);
* include external language links (.ss-lang) into the navbar;
* provide a company widget (.ss-company).

Split AutotocHeader.ss from Autotoc.ss to simplify overriding.