In the vast majority of cases, people want poll voters to be public. Previously, the checkbox for this was hidden behind the 'show advanced' settings in the poll builder UI.
This commit makes three changes to improve the experience:
1. Add `public=true|false` to poll markup (previously it would only be added when true
2. Bring the 'public' switch outside the 'show advanced' section for improved visibility
3. Change the default to 'true'
