[951] Make license consistent across CKAN

* Replace most instances of licence with license. The instances in js
  vendor files have been ignored.
* Setup clean_action_name to convert licence to license.
[#951] Fix more instance of licence, deprecate c.licences

* Fix instances of licence spelled in Title case.
* Add deprecation warning for c.licences.
[#951] Import deprecate_context_item in plugins.py
Merge branch '951-make-license-consistent'