adding initial participating groups

[danbjoseph]

• added hot and american red cross to the participating groups
• a few other minor edits and cleanup
• created a default-twitter-card.png in the style of the osmgeoweek one, though i'm not sure when it comes into play

@mikelmaron not sure about some of my "cleanup" please check and also let me know if i messed up when editing for a jekyll page