Added PeriodConverter to convert a period specification to a period, and applied it in the URL patterns.

mia_core/period.py

@@ -102,6 +102,10 @@ class Period:
     def description(self):
         return self._period.description
 
+    @property
+    def error(self):
+        return self._period.error
+
     @staticmethod
     def _get_last_month_start():
         """Returns the first day of the last month.

mia_core/templatetags/mia_core.py

@@ -86,7 +86,7 @@ def url_period(context, period_spec):
         request.resolver_match.app_name,
         request.resolver_match.url_name)
     kwargs = request.resolver_match.kwargs
-    kwargs["period_spec"] = period_spec
+    kwargs["period"] = period_spec
     return reverse(view_name, kwargs=kwargs)