Add <meta> social image on Event Pages (to improve social sharing)

[domlet]

Optimize how event pages appear when shared on Facebook.

Here's how they appear now:

![image of facebook post] (https://www.dropbox.com/s/whwgnzrlhovxb9s/railsbridge_event_fb_share.png?dl=0)

I think this is how to add an image (posted 3 years ago):
https://www.facebook.com/help/community/question/?id=10200814000150949

[domlet]

I nominate this image:
https://www.bridgetroll.org/assets/troll_plus_logo-10175e4141eb605fb0a96d68885ab9c0beefb987e4dbdb837b51937e5f295969.svg

[ultrasaurus]

I love the troll, though there's a parallel visual design / branding effort underway, where we planned to change the name of the website to events.bridgefoundry.org and keep bridgetroll as the name of the open source software project (#488).

Any ideas for an alternate image?

cc @shastablasta @LauraKirby