Restore body block to base template.

Commit b4c7717cb80cb13a2f13aae8855e226685306880 refactored the login template to override the entire body of the base template, and added a `body` block in base.html to make that possible. However, a subsequent merge seems to have removed those block tags from base.html, with the result that the login template does not render at all.

This PR simply restores the block tags.