Report the date of the first submission of participants

[reprogrammer]

It would be nice to report the duration of the time that our participants have used CodingSpectator. The script scripts/summarize-user-activities.rb reports the date of the creation of the account and the last data submission. It would be nice to make the script report the date of the first data submission of each participant as well.

[Wanderer777]

I think that a more precise measure of the duration would be to get the timestamps of the first and the last operations recorded by CodingTracker.