Use database transactions

[riggraz]

There are a few places where multiple database operations that must be executed together are not under transaction:

• Automatic insert of Follow when creating a new Post (posts_controller.rb, action create)
• Automatic insert of PostStatusChange when updating a Post (posts_controller.rb, action update)

Put these operations under transaction so that, if one of them fails, everything is reverted back. Consider putting the logic in a workflow (see app/workflows for examples) to avoid cluttering the controllers too much.