Resolve "Statistics seem to be based only on requests created in the last hour"

What does this mr do?

Changes the average waiting time to be calculated over the handled requests in the last hour instead of the handled requests that were also created in the last hour.

Also changes the active assistants to be calculated over the handled requests in the last hour instead of the handled requests create in the last hour.

Actions taken to fix bug

Change the time from getCreatedAt() to getHandledAt()

Does this MR meet the acceptance criteria?

• I have added a changelog entry to reflect the significant changes I made and the bug I fixed.
• A test was created to test the bug.
• I have updated the documentation accordingly.
• I adhere to the style guide.

Closes #194 (closed)