Fixed rewrite rules for Mediawiki

If Mediawiki was served from the root directory of the server
it was impossible to serve other directories.
    
Make sure that URLs defined in servedDirs are not rewritten.
Use case: serving local copy of MathJax:

````
  mathJax= pkgs.fetchgit {
    url = https://github.com/mathjax/MathJax.git;
    rev = "1c54436ae6166a91495aba8a98cc18816b4789df";
    sha256 = "1dgh02y1rkn967pgyhbczqby7lvw9kqi7jmdrb2lfgjbkqrh5arv";
  };

  mathJaxUrl = "/mathjax";

  mathJaxExt = pkgs.fetchgit {
    url = https://github.com/zalora/Mediawiki-MathJax.git;
    rev = "880adf7f9da55dbe257043fe431f825211ee96e1";
    sha256 = "17s3pbxj6jhywsbdss1hqmss8slb89jkwirlsbd0h16m130q72n8";
  };

...

  services.httpd = {
    enable = true;
    servedDirs = [
      { urlPath = mathJaxUrl; dir = "${mathJax}"; }
    ];
    extraSubservices =
    [
      { serviceType = "mediawiki";
        siteName = "My Wiki";
        enableUploads = true;
        uploadDir = "/var/lib/mediawiki";
        urlPrefix = "";
        articleUrlPrefix = "";
        extraConfig = ''
          require_once("${mathJaxExt}/MathJax.php");
            MathJax_Parser::$MathJaxJS = '${mathJaxUrl}/MathJax.js?config=TeX-AMS-MML_HTMLorMML';
        '';
      }
    ];
  };

````

Also reintroduce `concatMapStringsSep` and `concatImapStringsSep`. This the second time I need it :-)

@shlevy ?