Mako is why we cannot have nice things

Workarounds did not defeat mako’s limited inheritance in the end.
Wanted to hide this a bit deeper but exposing it could not be avoided.
Clean up accidental paste
${self.body()} is leaner than <%block name="content"/>

Primary motivation: theming becomes less scary without the content
block. It is less intimidating when it is “just HTML”. It is also
faster.

Not at all backward compatible. Theme authors will have to update
their themes for v7.0 anyways. This is the best time to do this.
Could include <%block name="content"/> underneath body() in
main.tmpl to retain some kind of compatibility, but as html_head()
is also gone there really is no clean backwards compatibility.