Add link to 2020 videos for old events

[reshamas]

@purna135

Suggestion:
On the events page (https://pymcon.com/events/), where there is:

• All events
• Upcoming events
• Past events

Add another button:

• 2020 events = = > link to the 2020 playlist: https://www.youtube.com/playlist?list=PLD1x-BW9UdeHN2vwR6kIApJATd2jZzeya

Goal: drive more traffic to other videos

[purna135]

Thank you; this will be really great :) I will open a PR soon