Add twitter feed to sidebar

Adds a twitter feed to the website sidebar.  The default is for @hollowearth, but on program pages, the feed will change to the program's twitter feed.   

There is an addition of twitter_id to the program DB/model

One thing which has changed is the Schema.db.  I don't know why there is a deletion of the tables for active_admin_comments and admin_users.   I didn't do anything to remove them when I was doing work on this change.