Copilot payments: Code quality

[jmgasper]

A few issues to clean up:

This flag should be a boolean:
closed: { type: String, required: true, default: "false" }

Should check here for invalid challenge ids:

async function getChallenges() {
... 

const challenges = _.get(response, 'data.result.content');
    logger.debug('Successfully retrieved challenges.');
    return challenges;
  } catch (err) {

 async function getChallengeById(id) {
...  const challenge = _.get(response, 'data.result.content');
    return challenge;
  } catch (er) {
    throw errors.convertTopcoderApiError(er, 'Failed to get challenge details by Id');
  }

Everywhere in file names, you use the term 'Payment' - This should be 'CopilotPayment' so it's fully clear that this is not referring to member payment