Fix history on delete comments
Merge pull request #71 from taigaio/remove-fk-from-history

Fix history on delete comments
Add specific order fields for better handle of order in diffent sections.
Implemented the ability of mark some history entries as hidden.

Snapshots with order fields (that are considered not important
for preview) are automatically marked as hidden.
Add migrations for order and history model changes.
Fixed strange behavior on make_diff function (possible bugfix)
Add separated methods for bulk update order for: kanban, sprint and backlog.
Fix/Improve tests for new changes introduced in bulk order update process.