[#1193] Deprecate helpers.get_action()

Fixes #1193
Merge branch 'master' of github.com:ckan/ckan into 1193-deprecate-get-action-helper-second-try
[#1193] Move long deprecation message to changelog

Also add "deprecated" to docstring, needed to prevent warnings and test
fails.
[#1193] Improve doc on how to deprecate functions
Merge pull request #1727 from ckan/1193-deprecate-get-action-helper-second-try

[#1193] Deprecate helpers.get_action()