Add social media sharing function on the site

we'd like to provide two kinds of sharing functionality:

- by providing share via fb, twitter, g+, snapchat, whatsapp on every page of the site to encourage users to share the data. Here's an example: http://codefornepal.org/en/2017/08/nepalflood/

- another sharing functionality would be when a user is simply trying to leave the page/site - she will be prompted to share the page with others with the message "Help your friends find data about their place by sharing NepalMap with them."