Remove unused Guest#create_without_name_validation

* Remove unused Guest creation method
* In InviteeBuilder, use `Guest#create` instead of unused method
* Cleanup and refactoring in the course of bug investigation
* Test guest.has_ever_logged_in defaults to false
* Add missing validation for has_many :votes
* Add tests for Guest name validation
* Extract InviteeBuilder#create_yammer_user method

Minor
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* Fix test descriptions when inaccurate
* Use more verbose validations and one per line for clarity
* Use private method name to describe whitespace filtering test instead of more general 'filters'
* Add clearer context blocks for InviteeBuilder `if` tree
* Use single quotes when not interpolating
* Break lines running over 80 chars
* Sort attr_accessibles in Guest
* Sort validations in same order as in model