Allow expired lock on api requests

When renewing or deleting (and perhaps getting?) a lock via the api an expired
lock should be allowed.
References issue rparent#6. See comment
rparent#6 (comment)
To enable this get_valid_lock_token_or_error now uses the new
LockManager.get_for_contenttype_and_id. (LockManager's get_for_object now also
uses this new method so there's no code duplication)
Via the parameter allow_expired it is possible to exclude expired tokens,
though the default is True.

lock_tokens/managers.py

@@ -8,9 +8,12 @@ class LockTokenManager(Manager):
 
     def get_for_object(self, obj, allow_expired=True):
         contenttype = ContentType.objects.get_for_model(obj)
+        return self.get_for_contenttype_and_id(contenttype, obj.id, allow_expired)
+
+    def get_for_contenttype_and_id(self, contenttype, object_id, allow_expired=True):
         lookup_fields = {
             'locked_object_content_type': contenttype,
-            'locked_object_id': obj.id
+            'locked_object_id': object_id
         }
         if not allow_expired:
             lookup_fields['locked_at__gte'] = get_oldest_valid_tokens_datetime()

lock_tokens/views.py

@@ -8,7 +8,6 @@
 from lock_tokens.exceptions import AlreadyLockedError
 from lock_tokens.models import LockToken
 from lock_tokens.settings import API_CSRF_EXEMPT
-from lock_tokens.utils import get_oldest_valid_tokens_datetime
 
 
 def lock_tokens_csrf_exempt(view):
@@ -35,12 +34,12 @@ def get_object_or_404(self, app_label, model, object_id):
         except contenttype.model_class().DoesNotExist:
             raise Http404("The object with id %s does not exist" % object_id)
 
-    def get_valid_lock_token_or_error(self, app_label, model, object_id, token):
+    def get_valid_lock_token_or_error(self, app_label, model, object_id, token, allow_expired=True):
         contenttype = self.get_contenttype_or_404(app_label, model)
         try:
-            lock_token = LockToken.objects.get(locked_object_content_type=contenttype,
-                                               locked_object_id=object_id,
-                                               locked_at__gte=get_oldest_valid_tokens_datetime())
+            lock_token = LockToken.objects.get_for_contenttype_and_id(contenttype,
+                                                                      object_id,
+                                                                      allow_expired)
         except LockToken.DoesNotExist:
             raise Http404("No valid token for this resource.")
         if not token == lock_token.token_str: