Charities can now add social network links to their charities

Added 4 social image icons courtesy of
https://github.com/neilorangepeel/Free-Social-Icons/

Added styling for the new social icons

Updated the sidebar view for charities to show new icons

Created form view for creating/editing social links

Created migration for social_links table

Created Model for social_links table

Created SocialLinkController to handle requests for the SocialLink model

Created SocialLinkValidator to validate multiple updates for SocialLink model