Embed Google Calendar plugin

[cpradio]

@zogstrip fixed the one-boxing recently

Can Google Calendar be One-Boxed?

Sorry about the delay, just pushed a fix (in 2 parts) https://github.com/discourse/onebox/commit/fd1b0bd93481dc51e6da86202065b664a7705b71 https://github.com/discourse/discourse/commit/e3f8182125fbf853ff04288bb0dbb9972627f184