Fixing Medium's buttons

A couple of quick recommendation for fixing some of the issues with Medium's buttons - some specific to Medium, others are more general problems with flat user interfaces:

- Corrected alignment and padding of "Following" label - Increased font-weight for contrast - Balanced chevron with font-weight (not sure if it's just me but the optical alignment is still slightly off - no doubt could be solved by point pixels but I'm not going to cheat) - Introduced affordance into the button with defined border and shadow