Move middleware sources to `server/middleware`

The new location allows developer to use the following identifiers
when loading the middleware using the new declarative style:

    app.middlewareFromConfig(
      require('loopback/server/middleware/rest'),
      { phase: 'routes' });

    app.middlewareFromConfig(
      require('loopback/server/middleware/url-not-found'),
      { phase: 'final' });

/to @raymondfeng @ritch Please review. The convention introduced here should become the standard for all loopback components.