Refactor authentication_pages_spec valid_signin code

- Add the code to spec/support/utilities as a helper method
Refactor: authentication_pages_spec

- Add a new matcher for asserting on heading messages
Refactor: authentication_pages_spec

Create new matchers to stop testing implementation and start testing
behavior instead.
Refactor: user_pages_spec

- Add use of new matchers for headings and titles
Refactor: Move matchers to request_matchers

Trying to give a better structure to where things are with proper names