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' });
Merge pull request #793 from strongloop/feature/move-middleware-to-conventional-location

Move middleware sources to `server/middleware`