Closes #47.

Almost too easy. Redundant `ApplicationHelper::BuildUserFor:MFBUser`
class now gone, replaced by `User` entity. (Technically, by whatever is
returned by calling `CCO::UserCCO.to_entity(current_user)`, but it meets
the current need.

RSpec: 401 examples, 0 failures
RuboCop: 111 files inspected, no offenses detected