Refactor checkouts app

Change "checkin" usage to "check_in".

Update URL parameters to the "_id" form.

Add support to checkout and check in multiple documents.

Optimize queries that used an ID list of documents for
filtering using values_list('pk', flat=True). These
queries now use .values('pk') as a subquery.

Add pre save hooks to block new document version uploads.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/checkouts/hooks.py

@@ -0,0 +1,13 @@
+from __future__ import unicode_literals
+
+from django.apps import apps
+
+
+def hook_is_new_version_allowed(document_version):
+    NewVersionBlock = apps.get_model(
+        app_label='checkouts', model_name='NewVersionBlock'
+    )
+
+    NewVersionBlock.objects.new_versions_allowed(
+        document_version=document_version.document
+    )