Open social media links in new tab and add Medium icon (#143)

* Open social media in new tab and add Medium

svg file pulled from Medium post on their logos
and brand guides dated Aug 22, 2017

https://medium.design/logos-and-brand-guidelines-f1a01a733592

* Add new-window accessibility icon for social media icons

Visible on-hover of link-transition class, this PR also
adds a description to the social media icons indicating that
they will open in a new window.

This way, users will be able to see and hear (via a reader)
when a tab will open in a new window.

This approach is based on the one outlined here:
https://medium.com/@svinkle/why-let-someone-know-when-a-link-opens-a-new-window-8699d20ed3b1