Add interest form to the article sidebar

[Scotchester]

Along with some general CSS cleanup along the way, plus a few under-the-hood Jekyll updates:

• Clean up old baseurl/assetpath junk
• Clean up exclusion list in _config.yml
• Update Gemfile to latest recommendation from https://jekyllrb.com/docs/github-pages/
• Remove Gemfile.lock from repo and add it to .gitignore per guidance from http://ben.balter.com/jekyll-style-guide/gitignore/

Review:

• @ielerol

[marteki]

Showing up in the sidebar! I wonder if it makes more sense to have the employment interest form above the article archive list rather than below it, but that's a conversation that can wait until after this is live.

:+1: for merge.

[Scotchester]

Thank you, @marteki!