Avoid using the theme when not required

Do not forcibly include the default theme. This makes overriding easier
as you do not need to override Bootstrap.ss to use a custom theme: just
include your custom theme (e.g. a Bootswatch theme) after it.
Content: put the title above the TOC

When using autotoc, vertically align the table of contents to the
content instead of the title. This will keep the title above it.
Content: split in ContentHeader and ContentBody

This subdivision makes easier the overriding of the page content. For
instance, my silverstripe gallery module can override only Page.ss to
include the gallery between header and body.
Implement a proper editor.css

Bootstrap does not follow SilverStripe conventions, so there is no
.typography class there. Port a substed of the relevant CSS into
editor.css to have a (basic) working typography in the CMS.
Remove hand-crafted caption on carousel

I do not remember why I put that crap in the CSS: it breaks the caption
on the standard Bootstrap 3.1 carousel. Removed to be able to use it
again.