Update index pages and add version info

This PR does a couple of things:

Add git_info jekyll plugin
Update wallscreen index page to be more useful
Update homepage to add version footer

Previously we had a page for each wallscreen that you could navigate to, but it didn't list anything useful, because the data didn't match the template. Now, if you click one of the wallscreens on the homepage, you get a page that lists all of the experiences for that wallscreen, which uses the same design.

There's also now a small footer on both index pages that includes version and build info (although some of this info isn't available in netlify). The git_info plugin can be used to do the same thing for #229.

I think this will help (but not solve) both #229 and #213, since it adds some helpful features that are curator or developer-focused. Regular users will never see these pages, and we don't need to design for them, but being able to see version info and pick which experience you want to view is helpful.

sul-dlss / wallscreens

Update index pages and add version info #235