Ensure that there is at least one org manager per org

* The only exception is right after creation, in which there may be no
  organization managers.
* Rescue from Sequel::HookFailed exceptions in RestController::Base#dispatch

[#66448526]

Signed-off-by: James Myers <jmyers@pivotallabs.com>
Minor test refactoring

[#69544442]
Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>
Refactor stagings_controller.

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>
Finish StagingsController refactoring

[#69544442]

Signed-off-by: Kai Zhang <kaizhang@cn.ibm.com>