Validate translations in config/settings_schema.json

[macournoyer]

This is only about translations validate. See #70 for validating the format of the file.

Doesn't seem to be documented anywhere, but settings_schema.json accepts translations:

[
  {
    "name": {
      "cs": "Barvy",
      "da": "Farver",
      "de": "Farben",
      "en": "Colors"
    },
    "settings": [
      {
        "id": "colors_text",
        "type": "color",
        "default": "#232734",
        "label": {
          "cs": "Text",
          "da": "Tekst",
          "de": "Text",
          "en": "Text"
        }
...

We should do the same checks we do inside {% schema %}. See #62.