Changes to `lectures.quantecon.org` page

[John Stachurski]

@DrDrij this is from my email, just helping us keep track of what needs to be done:

I think I've gone a bit overboard with all the Sloan badges. On this page it eats up too much space:

https://quantecon.org/lectures/

Would be able to strip it out, as well as the QuantEcon badge?

[DrDrij]

• Sponsor badges removed from lectures page
• Added section to homepage with sponsor badges, heading "Sponsors"
• Section includes ANU and Sloan

PR: #12