Give users more control over URL shortening

Give users more control over URL shortening

Users and administrators can set how long an URL can be before it's
shortened, and how long a notice can be before all its URLs are
shortened. They can also turn off shortening altogether.

Squashed commit of the following:

commit d136b390115829c4391b3666bb1967f190a0de35
Author: Evan Prodromou <evan@status.net>
Date:   Mon Apr 26 02:39:00 2010 -0400

    use site and user settings to determine when to shorten URLs

commit 1e1c851ff3cb2da5e0dc3a0b06239a9d9c618488
Author: Evan Prodromou <evan@status.net>
Date:   Mon Apr 26 02:38:40 2010 -0400

    add a method to force shortening URLs

commit 4d29ca0b91201f6df42940297ed5b64b070efe49
Author: Evan Prodromou <evan@status.net>
Date:   Mon Apr 26 02:37:41 2010 -0400

    static method for getting best URL shortening service

commit a9c6a3bace0af44bcf38d1c790425a7be9c72147
Author: Evan Prodromou <evan@status.net>
Date:   Mon Apr 26 02:37:11 2010 -0400

    allow 0 in numeric entries in othersettings

commit 767ff2f7ecfd7e76e8418fc79d45e61898f09382
Author: Evan Prodromou <evan@status.net>
Date:   Mon Apr 26 02:36:46 2010 -0400

    allow 0 or blank string in inputs

commit 1e21af42a685f600f4a53f49a194013e78b12f20
Author: Evan Prodromou <evan@status.net>
Date:   Mon Apr 26 02:01:11 2010 -0400

    add more URL-shortening options to othersettings

commit 869a6be0f5779aff69018d02f9ac0273946040d9
Author: Evan Prodromou <evan@status.net>
Date:   Sat Apr 24 14:22:51 2010 -0400

    move url shortener superclass to lib from plugin

commit 9c0c9863d532942b99184f14e923fc3c050f8177
Author: Evan Prodromou <evan@status.net>
Date:   Sat Apr 24 14:20:28 2010 -0400

    documentation and whitespace on UrlShortenerPlugin

commit 7a1dd5798f0fcf2c03d1257a18ddcb9008879de0
Author: Evan Prodromou <evan@status.net>
Date:   Sat Apr 24 14:05:46 2010 -0400

    add defaults for URL shortening

commit d259c37ad231ca0010c60e5cfd397bb1732874a4
Author: Evan Prodromou <evan@status.net>
Date:   Sat Apr 24 13:40:10 2010 -0400

    Add User_urlshortener_prefs

    Add a table for URL shortener prefs, a corresponding class, and the
    correct mumbo-jumbo in statusnet.ini to make everything work.