remove twitter feed on homepage, replace with usage sidebar

[falquaddoomi]

The twitter feed on the homepage will be replaced with a selection of resources from the usage page in order to make them more visible. Specifically, there will be links to the different github repos for developers, links to the tutorials on ohmage/Mobility for researchers/individuals, and some resources for researchers that are yet to be decided.

[jshslsky]

Here is the link to the Mobilize wiki. https://wiki.mobilizingcs.org/

Also, if it is not already linked, the server wiki should be: https://github.com/ohmage/server/wiki/2.x-Documentation