[#1330] Get user ids from a list of ids or names
[#1330] Filter activity from specified users
[#1330] Update documentation
[#1330] Use actual value of site_id for default
[#1330] Fix test failures
pep8 fix for test failure
Update action function and documentation

* Split the helper function into two functions.
* Add better docstrings.
* Improve the documentation and explain about site user.
Fix tiny error.
Enable `hide_activity_from_users` by default
Merge branch 'master' of github.com:ckan/ckan into 1330-hide-activity-stream

Conflicts:
	ckan/logic/action/get.py
	ckanext/textpreview/tests/test_preview.py
Merge branch '1330-hide-activity-stream'