Django admin usability improvements

Copied from PR #63 by @bryan-brancotte.

Author: adamalton

cspreports/admin.py

@@ -6,9 +6,12 @@
 
 
 class CSPReportAdmin(admin.ModelAdmin):
-    list_display = ("id", "created", "document_uri", "blocked_uri")
+    list_display = ("id", "created", "document_uri", "blocked_uri", "is_valid")
     fields = ("created", "modified", "json_as_html")
     readonly_fields = ("created", "modified", "json_as_html")
+    search_fields = ("json",)
+    list_filter = ("is_valid",)
+    date_hierarchy = "created"
 
     def json_as_html(self, instance):
         return "<br />" + instance.json_as_html()