RA-79: refactoring in BasePage, add Page interface, add assertPage metho...

1. I did some refactoring in AbstractBasePage to remove duplication.
2. I added a Page interface.
3. I added assertPage() method in TestBase and implemented it using the url path. I'm not 100% sure I like the impl, but I definitely think an assertPage method is a good thing and should be used often in tests.