Data generator script doesn't return stable DataPublication dates

[louise-davies]

Description:
The icat_db_generator.py script passes the -15y start date variable to DataPublication publication dates and to DataPublicationDates. The problem then is that given the same seed on different days, the data will be off by the difference between the two days. This causes problems with generating data for e2e tests.

Acceptance criteria:

• DataPublication publicationDate and DataPublicationDates no longer defined relatively, and are instead defined in a way that is fully reproducible with the same seed no matter the date the script is ran