variant: datauri for Django Pipeline

By default, Django Pipeline removes spaces from Data URIs used in
stylesheets. This causes a problem when you use SVG markup in your
Data URIs, as Bootstrap does for its mobile navbar toggle icon.

The Django Pipeline docs suggest that `variant: datauri` fixes this:

https://django-pipeline.readthedocs.io/en/latest/configuration.html?highlight=variant#variant

Thanks also to @suxilog for identifying and suggesting this fix:

jazzband/django-pipeline#743

Fixes #462.

proj/settings.py

@@ -109,10 +109,12 @@
         "caps": {
             "source_filenames": ("caps/scss/main.scss",),
             "output_filename": "css/caps.css",
+            "variant": "datauri",
         },
         "scoring": {
             "source_filenames": ("scoring/scss/main.scss",),
             "output_filename": "css/scoring.css",
+            "variant": "datauri",
         },
     },
     "CSS_COMPRESSOR": "django_pipeline_csscompressor.CssCompressor",