Ars control add to cart button

[ancasimon]

This PR adds toggling between buttons in a couple of places depending one whether user is logged in or not.

To test:

• To check Log In / Log Out toggle:

  1. Log out of the app.
  2. Go to the Login page. Expected result: You should see only the Log In button and you should also see the registration fields.
  3. Log in.
  4. Go back to the Login page. Expected result: You should see only the Log OUT button and you should NOT see the registration fields.

• To check Add to cart / Please log in buttons:

  1. Log out of the app.
  2. Go to the Single Product View page for any product. Expected result: You should see the Please log in .... button.
  3. Log in.
  4. Go back to the Single Product View page. Expected result: You should see the Add to Cart button.

[Nikababy01]

@ancasimon looks great!