Link to data bucket in reports

We don't link to data buckets in reports.
This would be helpful since it would allow people to do their own processing on corpora/output directories, and is generally useful for transparency.
The quick and dirty way to do this is to from the report page do `'http://commondatastorage.googleapis.com/fuzzbench-data/' + $EXPERIMENT_NAME`
Cleaner way is to save the data directory when we create the experiment in the database.