Update active users definitions

[faucomte97]

Summary
Update definitions of active users based on new decisions (active users by year, returned in the last year, etc)
Add a definition for retained users

We are now recording active yearly rather than using the 3 year rule.

Deadline
When does the task need to be completed by? Specific or rough date. Tip: Don't just put ASAP!

Links
Refer to this doc

Reviewer
@lauracumming

People to contact
@lauracumming