fix: invalid key reference in retrieving form submissions

[karrui]

Problem

searchSubmissionsForForm aggregate pipeline step was incorrectly matching against a key of key instead of the key parameter passed into the function:

let searchSubmissionsForForm = (key, formId) => [
  {
    $match: {
      // key "key" is matched instead of the key parameter.
      key: mongoose.Types.ObjectId(formId),
    },
  },
]

This PR fixes that by correctly using the given key parameter as a match key:

let searchSubmissionsForForm = (key, formId) => [
  {
    $match: {
      [key]: mongoose.Types.ObjectId(formId),
    },
  },
]