Introduce available items, a context for rendering the items page

--HG--
rename : webapp/src/test/java/test/support/org/testinfected/petstore/web/MockPage.java => webapp/src/test/java/test/support/org/testinfected/petstore/web/LegacyMockPage.java
rename : webapp/src/test/java/test/support/org/testinfected/petstore/web/OfflineRenderer.java => webapp/src/test/java/test/support/org/testinfected/petstore/web/LegacyOfflineRenderer.java
Use a view model to render checkout page, I feel much better about the code

--HG--
rename : webapp/src/main/java/org/testinfected/petstore/helpers/Messages.java => webapp/src/main/java/org/testinfected/petstore/Messages.java
rename : webapp/src/main/java/org/testinfected/petstore/helpers/ListOfErrors.java => webapp/src/main/java/org/testinfected/petstore/views/ValidationErrors.java