Add new standard layout 'all' (in addition to single,list)

[bep]

This relates to #13543 and #13541 and #12988.

While it's possible to use rss.xml and html.html as a somewhat "catch all", it would be great to have a layout that "covers everything", e.g.

layouts/all.html

I'm open to better naming suggestions, but I think the above looks great.

[tycobra]

Here are some suggestions.

1. default.html
2. base.html (might conflict with baseof.html)