Add comments from pull request
Add documentation for reports
Use intervals instead of daterange
Refactor active length
Merge branch 'admin-reports' into admin-reports-docs

Conflicts:
	corehq/apps/hqadmin/reporting/reports.py
Revert "Use intervals instead of daterange"

This reverts commit 5f619ba79cb92c1f17fc76904abe451db9a1b0d0.

Conflicts:
	corehq/apps/hqadmin/reporting/reports.py
Merge pull request #4449 from dimagi/admin-reports-docs

Admin reports docs