Fix diazo rules path

If the diazo rules path isn't forced to use the file protocol, diazo
first tries to resolve the rules files (rules.xml, rules_notheme.xml,
rules_header.xml etc.) from the Adhocracy server. As these paths do not
exist, Adhocracy returns the /error/document page with status code 404
and Diazo then tries to find the file in the filesystem. In most themes
this results in 4*2=8 unsuccessful calls to Adhocracy on system startup.

Therefore system startup performance is increased significantly on
systems using diazo theming.

Note that this would finally fix the "No such instance: localhost,
defaulting" warnings which were only silenced in
94c74831e3992af212ac2c59f7ad9ec947cd82ae.
Merge pull request #882 from nidico/feature-fix-diazo-path

Fix diazo rules path