Add additional hook to delete_model allowing action between session.delete and session.commit

[cancan101]

I suggest adding an additional hook to delete_model allowing actions between session.delete and session.commit.

Currently, delete_model looks like:

flask-admin/flask_admin/contrib/sqla/view.py

Lines 1132 to 1134 in 7fa26ab

	self.session.flush()
	self.session.delete(model)
	self.session.commit()

which means that if you cannot first attempt to delete locally (and then flush to make sure it works) before calling an external service and then finishing with a commit.

Ideally there would be a new hook:

 self.session.flush() 
 self.session.delete(model) 
 self.before_model_delete_commit() # new hook
 self.session.commit() 

This allows getting e.g. integrity errors before I make an external call since I cannot roll back the external call.