Add ability for CLA bot to comment on PRs when user has not signed CLA

- [x] Use the github API to create a comment on a PR. Use e.g. the cla-bot-playground repo to test this.
- [x] Use the github API to retrieve comments on that PR to see what the payload looks like.
- [ ] Comment when the user has not signed the CLA _and_ the comment has not been made previously.
- [ ] Comment after the user has signed the CLA _and_ the comment has not been made previously.

constraints:

- need to make sure the bot doesn't comment the same thing multiple times on PRs (as the PR check gets set multiple times in a typical interaction). only one comment of each kind (i.e. 'sign the cla' or 'thanks for signing the CLA').
- worth noting that the typical listcomments API call returns 30 comments max usually.

Start with the existing GitHub API work and use the cla-bot-playground repository to exercise pull-request comments and comment retrieval. Implement one comment for each CLA state, avoiding duplicates when checks run repeatedly and accounting for the comments API limit. Done means both unsigned and signed contributors receive the appropriate comment only once.

Written by the indexing model from the issue text.