Add external link icon to nav button

[kristinashu]

Add external link icon to nav button when the link is external.

@sabrinang can you please provide an svg of the icon?

Comp:

Prod:

[sabrinang]

Let's go with external icon with full square (looks better at small size too) and use at 16px inline with buttons.

Comparison with alternative icon:

Exported Black/White svgs here:
https://drive.google.com/open?id=1sc9am1e190ICxW2MbcWgvVoB95gtrXDy

[kristinashu]

"Submit Proposal" is now an internal page so we won't need the icon. But let's keep this open in case we need it for a different CTA. @sabrinang can you think of a future CTA button that would need it?

[sabrinang]

I'm assuming tickets will stay internal and schedule, speakers pages have yet to be created but hoping they are internal as well! Not quite sure where the schedule will live

[kristinashu]

Ok awesome, let's close this then!