Add logging for the data submitted

[desrosj]

To help hosts understand what data was submitted, the values will now be saved into a log file.

[peterwilsoncc]

A few notes:

• It would be good to generate the log independently of whether the reports are submitted to wp.org or not. That will allow contributors to retain log files. This will require moving the log generation in the plugin as it's currently in the submission function.
• Blocker: Log files are best stored in the bash scripts directory rather than in the mu-plugins directory.
• With above, the .gitignore file ought to be updated to exclude the log files.

[peterwilsoncc]

This is now recording the log file for me, thanks. For testing without submitting, I'm setting the reporter url to https://invalid.invalid which creates the log but doesn't submit, of course.

If moving the log file is too fiddly then I think it's fine to keep it in the mu-directory as this isn't intended to be run on a production machine.