Internal: Fix study start date calculation

Updater should now calculate the start date of the study right element in a more reliable way.

Previously there were a lot of Sentry errors that weren't really errors from user's point of view. These errors happened if the user didn't have a right to see tags. The tags were requested at least on the population statistics view. Previously the backend responded with 403 that caused the error, now the response is null with a status code of 200 if the user is not authorized to see the tags.