Support embedding a group calendar

[r-a-y]

Users should be able to embed the group calendar on other sites.

Update:

Appending ?embedded=true will remove the header and footer on group event pages:

• example.com/groups/GROUP/events/?embedded=true
• example.com/groups/GROUP/events/upcoming/?embedded=true

This will allow for embedding with an IFRAME.

Todos:

• [x] Override wrapper group template to remove the header and footer (commits e21ae70 / 438960c)
• [x] Create a specialized widget plugin that will embed an IFRAME of the group calendar on sub-sites (commit 4f40d69)
• [x] Support shortcodes and Shortcake plugin