Add endpoint to get the definition of a spare

[joelvdavies]

Requires #413. Endpoint should:

• Be a GET at /settings/spares_definition
• Return the list of usage statuses that are classed as spares e.g.

  {
   "usage_statuses": [
     { "id": "6723a734ae807034e53cdd13", value: "In Use" },
     { "id": "6723a734ae807034e53cdd14", value: "Using" }
   ]
  }