Include JSON data in the feedback comment

This takes selected statistics for testing runs and puts those into the MR comment (as JSON, base64 encoded). This will allow us to retrieve these statistics later and compare with production statistics. See #7 (closed).

We use base64 encoding here to make this a tad bit easier to parse in the HTML comment. See !33 (comment 601642771) (edit the comment to see the HTML comment).

Closes #7 (closed).