social broadcast-formats

see #137 
- [x] add facebook url to schema
- [x] add blog url to schema
- [x] add attr_accessors
- [x] show facebook icon if present
- [x] show wordpress icon if blog url is present
- [x] validate url for facebook_urls
- [x] validate url for blog_url
- [x] add blog url input field to format create view
- [x] add blog url input field to format edit view
- [x] add facebook url input field to format create view
- [x] add facebook url input field to format edit view
- [x] add already existing blog urls #106 
- [x] add already existing facebook urls #133 
- [ ] deploy