Generate social media images automatically

[benabraham]

Automatically generate social media images for talks and workshops.

Idea (details are open to change) is that the following will run on every deploy or is invoked manually (probably better):

1. HTML pages at pycon.cz are are going to be screenshoted using 3rd party service like https://apiflash.com
2. Screenshots will be added to talks/workshops.
3. Using FB and Twitter API pages with changed screenshots will be invalidated.