Fixes speakers on all screens

Adds the notion of a very large screen, to separate between 13"-15" desktops and above.

Sferik's name is really big, so it was getting on top of the talk description and on 13" inch monitors the landing page speakers part was rendering with 4, instead of 3.
Is this saying the following?
- margin 0 below large size
- margin 3em between large and very large
- margin 9em for very large and above

if so, wouldn't it be more clear like this?
```sass
margin-left: 0
@media all and (min-width: #{$screen-lg-min})
  margin-left: 3em
@media all and (min-width: #{$screen-vlg-min})
  margin-left: 9em
```