Enhance extensibility of the siteconfig settings template.

The siteconfig settings template has some new blocks that allow for
better overriding of content, especially when used through the
configure_extension.html template.

The page title can now be changed without overriding the entire title
tag. There's a block for the particular page's subtitle, separate from
the page-wide title block.

The buttons box at the bottom of the page can be replaced or augmented
as well. The Save button can be replaced, and pages can add additional
buttons wherever they wish.

Reviewed at https://reviews.reviewboard.org/r/6581/