Renamed `AllTests` to `AllModelTests`.

I am making another test suite called `AllViewTests`, so I need a more
distinct name for the test suite covering `Model` and its subclasses.
Updated the HTML doctype declaration to HTML5.
Created the `User` and tested `User.create()`.

* Created the `User` subclass of `Model`.
* Implemented the abstract methods inherited from `Model`.
* Built a unit test to test `User.create()`.
* Added the `UserTest` unit test case to the `AllModelTests` test suite.
Implemented `User.read()`.
Implemented `User.update()`.
Implemented `User.delete()`.
Added a description of the `Model`.
The contents page now checks whether the visitor is logged in.

* Created the contents page.
* Added methods to `User` for checking whether the website visitor is
logged in.
* The contents page displays “Login” if the visitor is logged out and
“Logout” if the visitor is logged in.
* The links to log in or log out are not yet implemented.