Strip embedded images from descriptions

[kwisatz]

Migrating tickets that have images embedded in their description now produce this:

      ["description"]=>
      array(1) {
        ["raw"]=>
        string(69) "!https://progest.youth.lu/attachments/227/screenshot.png!

{F3605720}"
      }

We can strip the string enclosed in exclamation marks since any attachments automatically get embedded.