Exercise 5.6.1: Use 'have_selector' instead of 'have_content' for a more precise test case

Change-Id: I4d41837de2d81b04030ff5a44ba286af2d1dd52a
Exercise 5.6.2:  Using an RSpec shared example to eliminate test duplication

Change-Id: Id1b578d12976c5751679a25158f6f2e7b8ea89ad
Exercise 5.6.3: Test the links point to the right pages

Change-Id: I7dd8a7db2ae95b8c42a6299a03ab43f015f61bf8
Exercise 5.6.4: Remove repetitive code in 'app/helpers/application_helper.rb' & 'spec/support/utilities.rb'

Change-Id: If39449605bc46e6d3208f8f26c7e0e59f4ea6bc1