Add config setting to let local Organization admin users set Public/Private

[reedv]

Ie. a config setting like..

#ckanext.publicrestrictiondatasets.allowed_users = sysadmin
ckanext.publicrestrictiondatasets.allowed_users = admin

to let users decide if they want sysadmins or local Organization admins (in addition to sysadmins) to have control over dataset visibility (rather than only admins).

[reedv]

see https://docs.ckan.org/en/2.9/extensions/plugins-toolkit.html#ckan.plugins.toolkit.ckan.plugins.toolkit.config on how to access the ckan.ini configs.

eg...

ALLOWED_ACCESS_LEVELS = ['sysadmin', 'admin']
access_level = toolkit.config.get("ckanext.publicrestrictiondatasets.allowed_users")
assert access_level in ALLOWED_ACCESS_LEVELS
.
.
.
if access_level is 'sysadmin':
    <do stuff>
else if access_level is 'admin':
    <do stuff>
else:
    <raise some error>