Add Condorcet count endpoint

[jb3]

Adds an endpoint, /forms/{form_id}/condorcet/{question_id} to perform a
Condorcet calculation for the newly added vote components.

Optionally, a ?winners= parameter can be specified to provide the number of
results to calculate for, defaulting to 1 winner.

The rest of the Condorcet table is provided.

An example response:

{
  "question": {
    "id": "vote-1",
    "name": "Vote for your preferred options",
    "type": "vote",
    "data": {
      "options": [
        "Dog",
        "Cat",
        "Ferret",
        "Worm",
        "Chris J Lovering"
      ]
    },
    "required": true
  },
  "winners": [
    "Chris J Lovering"
  ],
  "rest_of_table": {
    "Dog": {
      "wins": [
        "Cat",
        "Ferret"
      ],
      "losses": []
    },
    "Cat": {
      "wins": [],
      "losses": [
        "Dog",
        "Worm"
      ]
    },
    "Ferret": {
      "wins": [],
      "losses": [
        "Dog",
        "Worm"
      ]
    },
    "Worm": {
      "wins": [
        "Cat",
        "Ferret"
      ],
      "losses": []
    }
  }
}