Use `<time-formatter>` on homepage

[benfoxall]

We should use @philnash's <time-formatter> instead of the inline script we use on the homepage!

Found out about it via twitter

[omgmog]

We currently declare a formatted time in two places in the templates (1, 2). I don't think adding a JS dependency is the answer, but we could totally factor out a Jekyll include that does this 👍