Moves the site to Jekyll

[konklone]

I migrated the site to Jekyll!

• There are two layout files: default.html (for the main site), and bare.html (for the linking policy and thank you page).
• The bare pages now also have the same JS/CSS assets loaded, and same metadata.
• The thank-you page now redirects with JS instead of a <meta refresh> tag. The "you will be redirected" copy was moved up next to the main content.
• I rewrote the README and setup instructions.
• I made paths absolute (e.g. href="/js/18f.min.js" and not href="js/18f.min.js").
• Some paths changed: /thank-you/ instead of /thank-you.html. Same for linking policy. It seems early enough, and these pages not so important, that this should be fine.
• Fixed what I think is the cause of the mixed content warning we're currently getting at https://18f.gsa.gov.

This is based off of the devel branch, so some other un-deployed changes are present, such as removing Lena and the addition of a link to our open source policy.

Now that we're using absolute URLs, it means the site can't run behind a path prefix anymore (like our current staging area at 18f.github.io/18f-staging has been) -- and, we want to keep our options of using non-GitHub-Pages-supported Jekyll plugins open, so we need to use a non-GHP staging area. I will need to update our deploy server to set up a staging path and a production path.

Future development items

• Move site titles and descriptions (that appear in <meta> tags now) into the site's _config.yml.
• Move the blog posts from Tumblr into this site. Someone with Tumblr access needs to copy and paste the post data into markdown files and put them in a directory in this repo (or grant me access to do that), to start.
• Freeze or remove jquery.dotdotdot, and remove the use of git submodules.
• A Jekyll plugin could do the asset compilation instead of gulp, removing a dependency -- or assets could just be unminified.
• Team photo section should be generated by a team.yml data file - this will also let us refactor how we display teammates much, much easier.
• <rel = "canonical"> links at the top of pages.
• Use Travis to verify that jekyll builds without errors.
• Site's SSL cert needs to be regenerated as SHA-2.
• Site's SSL configuration needs an update.
• Point staging.18f.us at the same box, so we can use that as the staging URL instead of 18f.github.io/staging. (The staging repo can be deleted at that point, I believe.)
• Site needs to be moved out of GovCloud and into 18F's AWS public cloud.

But I don't think any of these are standing in the way of the merge -- the deploy details can be taken care of post-merge and out-of-repo, and the site here is as good as it is on https://18f.gsa.gov right now.