function doPlaceholdersMatch(strings) {
  // Technically placeholder `content` is not required to match by TC, but since
  // `example` must match and any auto-generated `example` is copied from `content`,
  // it would be confusing to let it differ when `example` is explicit.
  return strings.every(val => isEqual(val.ctc.placeholders, strings[0].ctc.placeholders));
}
Returns whether all the placeholders in the given CtcMessages match.
@param {Array<{ctc: CtcMessage}>} strings
@return {boolean}