Cross-link between sites, and refactor into multiple classes

[majelbstoat]

Hello @majelbstoat,

Please review the following commits I made in branch 'jamie-crosslink'.

c31713968990bab5d83db68cf486953ea161a009 (2015-10-04 19:08:56 -0700) Cross-link between sites, and refactor into multiple classes

R=@majelbstoat

MANUAL TESTING=lots of dubious posts

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[majelbstoat]

YOLOMerge PHP.