ClinEpi: add “Avg time to decision (days)” statistic to all studies table

[danicahelb]

see https://redmine.apidb.org/issues/43222

“Avg time to decision (days)” → amount of time between submission and decision to provide access or reject

decision was made to calculate the "avg time" statistic based on history logs (there is no need to take into account information from access requests made prior to the history log system being in place)

The data will come from the end_users table and the new history table.
Initially we thought of a new service endpoint but given that this information will be shown in existing pages:

• All Studies results table
• Study page
  Im hoping these will be new attributes/table in the study record xml (SQL)

More details:
avg_time_request_response_days:
1- is days enough granularity? YES (danica slack Jan 31 2021)
2- are we only considering transitions from request to another state? YES (danica slack Jan 31 2021)
----We may also want to consider time to 1st response as an additional stat