Make the display of social network links configurable on Petition thank-you pages

Currently the display of social network links is hard-coded in petition thank you pages. This should be configurable at the petition level.

1. add the is_share column to civicrm_survey
2. add alter table for 4.4 upgrades
3. add the checkbox field to create / edit petition form
4. modify thank you page class and template to assign and handle conditional inclusion of the social network tpl file