Return structured compliance information

Right now, we just return a single value of whether the PR is compliant (yes/no) and whether or not its CLA is externally-managed. It would be better to return structured information on a per-commit (or per-committer) basis, such that we could provide concrete feedback like:

> * :white_check_mark: committer1 (`c0ffee`) CLA compliant
> * :x: committer2 (`badf00d`) CLA non-compliant
> * :heavy_minus_sign: committer3 (`505050`) CLA externally-managed

Or even more detailed error messages, e.g.,

> * :x: committer2 (`badf00d`) is on the contributors list, but name on commit does not match CLA
> * :x: committer3 (`badc0de`) is on the contributors list, but email on commit does not match CLA

Then, users can just click on the commit to see what was the issue and fix it, without requiring manual inspection of all of the commits.

Of course, if the user amends their PR, CRBot should recompute the analysis and do one of the following:

1. post a new comment and delete the old comment (if posted as a comment)
1. rewrite the original message (might be confusing?)
1. post a new review (marking the old one outdated or resolved, perhaps)