Support "Step Challenge" challenge type

[SaintPeter]

Challenge type 7.

These have a unique structure:

description: [
  [
   "Image URL (Required)",
   "Image Description (Required)",
   "Text (Required), One line broken by <br />",
   "Click To Open URL (Optional)"
  ], 
  /* Additional Steps */
]

It seems like a nice interface would have single line for most of these, maybe a multi-line editor for the "text" item (description[2]) that would automatically create <br /> entries.

There would be an "Add New Step" button as well.