Add app-level sharing modal

* Rename YammerAutoCompleteMock to ScheddoYammerMock
* Extract footer partial

https://trello.com/c/KR3Pm23X
@wip Add styles from Reda's branch

* Merged from https://github.com/yammer/sched.do/compare/master...rl-ap-add-invite-page-to-new-event-flow
@wip
Add styling for guest/group delete button
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@wip
@wip
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