Add Tracker URL field to WordCamp details report

[timiwahalahti]

This is an attempt to add new "Tracker URL" field to WordCamp details report. Couldn't test, since csv export utility is in mu-plugins-private for some reason.

Warning: require_once(/usr/src/public_html/wp-content/mu-plugins-private/wporg-mu-plugins/pub-sync/utilities/class-export-csv.php): failed to open stream: No such file or directory in /usr/src/public_html/wp-content/mu-plugins/2-autoloader.php on line 27

Either that utility needs to be moved to a public repository, or this PR can act as a base for someone who has access to private plugins. I guess the latter one will be easier and faster.

Fixes #823

Props @peiraisotta

How to test the changes in this Pull Request:

1. Navigate to WordCamp Details report
2. Do csv export, the field is selected by default

[pkevan]

Either that utility needs to be moved to a public repository, or this PR can act as a base for someone who has access to private plugins. I guess the latter one will be easier and faster.

Turns out that the file in question is actually here: https://github.com/WordPress/wporg-mu-plugins/blob/trunk/mu-plugins/utilities/class-export-csv.php, but is built in a rather convoluted way (of which I don't know the history of). Is that helpful @timiwahalahti?