Update blurb about what a workshop is

Borrowed from our Tumblr about page.
Allow visitors to sign up for mailing list from the homepage

Instead of linking them to the MailChimp form.
Update middleman-dotenv

1.0.0 was yanked and is causing Travis build issues.
Rename "Civility Policy" to "Code of Conduct"

For exactly the reason mentioned in
https://github.com/railsbridge-boston/railsbridge-boston/pull/160
Added Typekit
Switched to Typekit's async embed
Somehow killed the head tag
Hide if font loading and supports js
Moved typekit.js into file
Removed hiding of text if font not loaded