PSGI entry points (#11632) -- add a plackMiddleware section to the config file; modify share/site.psgi to load from it.

This is minimal in implementation right now in that there's no support for passing args, which would generally be objects
mixed with strings, to the middlewares being added, as would be necessary if the .psgi files were to be largely migrated
to the config files.  I agree that it's important that upgrade scripts be able to add middleware (probably custom ones
that know how to get the session from %$env) but I'm not convinced of the utility of moving much logic out of
share/site.psgi.

share/site.psgi

@@ -56,6 +56,12 @@ builder {
         Plack::App::File->new(root => $config->get('uploadsPath'));
     };
 
+    # enable config defined Middleware
+
+    for my $mw ( @{ $config->get('plackMiddleware') || [] } ) {
+        enable $mw;
+    }
+
     # Return the app
     mount '/' => $wg->to_app;
 };