Attachments are not migrated

[cmungall]

This is an example:

      {
        "text": "screenshot of Protege explanation",
        "attachments": [
          {
            "url": "http://sourceforge.net/p/geneontology/ontology-requests/_discuss/thread/26af339c/af1f/attachment/Screen%20Shot%202015-05-15%20at%2012.15.59%20PM.png",
            "bytes": 170325
          }
        ],
        "author": "tairtb",
        "timestamp": "2015-05-15 19:17:43.701000",
        "last_edited": null,
        "slug": "af1f",

For a true migration, the attachment should be downloaded and stored somewhere, and the migrated ticket should point to the new location. Different groups may have different solutions: amazon, checking the attachment back into github (size limits...?), making this a hard ticket to have a general solution for