templates: clean up + upgrade to loopback-boot 2.0

 - Include `loopback-boot 2.x` in the root `package.json`

 - Include express 4.x middleware packages in the root `package.json`

 - Drop `email` datasource. It is no longer needed, since loopback-boot
   does not call `loopback.autoAttach`.

 - Rename model names in `rest/models.json` to reference the built-in
   loopback models. Make internal models like `AccessToken` not public.

 - Refactor the `server` template to use the boot-based registration of
   middleware (see loopback-example-full-stack).
Mark all `componentName` properties as required.