[MERGE] various: replace workflows by integrating them in models

Workflows are removed and replaced by python code directly embeded in model
files. There are several reasons to do this

 * most of workflows are simple state machines. They simple go from state to
   state under simple conditions. Short circuits exist allowing to bypass
   part of the flow, like cancelling a record. However it is easier to
   understand a flow composed of several simple methods than having to
   understand the workflow in a separate xml file
 * workflows are hard to debug and modify. Each modification require a small
   migration. Moreover ensuring records are not blocked or that there does
   not exist unreachable states is hard to do.
 * having all business code in models helps understanding the business code.
 * most of our flows are quite simple and does not require a complex machinery
   to reflect it

Workflow engine itself is kept. Lot of people use it and it can be very
useful in some cases, notably in modeling more complex flows. Removing it
is therefore not planned in this task.