One setup page

[ErinBecker]

This PR implements changes to the Social Science workshop overview page in conjunction with this RFC and the corresponding PR. The main changes are:

• Create a centralized Setup page with all install instructions and data downloads that learners will need for the entire workshop. To start this process, I've copied over install instructions from each lesson setup page and re-formatted them to be consistent with one another. The level of detail described in each setup page was different, so there is some work to do to make this read smoothly, but at least all the information is in one place now! (Note - for OpenRefine, I used @libcce's excellent instructions from the LC lesson as these were more up to date.)

• Updated the index page to share the same overall structure as other DC curricula.

Notes

• Since no workshops teach both R and Python, and many don't teach the SQL lesson either, no one learner needs all of the setup information currently shown on this page. @fmichonneau is going to build on this PR to have at least two separate URLs - both will have the core content, and will use some of the jekyll structure to include either the R or Python instructions. Learners will be directed to the correct page based on the value of the flavor variable in their workshop website.
• I also created a similar PR for the Ecology workshop overview page. I've copy/pasted the setup instructions for that workshop here, since all of the software needs are the same. Please let me know if I've missed any places that need to be different between these two curricula.