Closes #44.

Added new application helper function `#build_greeter_for`, in the mould
of `#build_menu_for`: it takes a user (entity or model) as a parameter,
and builds a "Hello, #{user name}!" message in a div suitable for output
to a view.

RSpec: 401 examples, 0 failures
RuboCop: 112 files inspected, no offenses detected