Polls: add property setting to enable/disable "public" setting

The Polls tool has a "public" access option (https://jira.sakaiproject.org/browse/SAK-18855) in the UI. But the ability to make a poll public requires that an institution take additional steps (javascript/Entity Broker/".anon" site role). Simply checking the "public" box in the UI will not result in a poll being publicly accessible.

Since this is the case, it seems like a property setting for the "public" option would be a good idea. By default, the value would be "false" and the "public" option would not display. If institutions want to do the extra configuration to make public polls possible, they can then enable display of the "public" setting.