Proper definitions of the terms in stats.

1.   Required - Issues that require release notes
1.a. done - issues having release notes done
1.a.1 done_empty - issue having release notes done but notes itself are empty.
1.b. todo - issues yet to have release notes to be put in
(Count 1 = Count 1.a + Count 1.b)

2.   Not required - Issues that doesn't require release notes
2.a  none - issue having release notes CF marked 'None' (not required)
2.b  issues with trackers where CF is not defined.
(Count 2 = Count 2.a + Count 2.b)

3.   Invalid - issues that has CF not done properly
3.a  nil - issues having CF values NULL or blank ('')
3.a is subset of 3.

4. Total - all issues
(Count 4 = Count 1 + Count 2 + Count 3)

Stats computation yet to be migrated in controller.
Still few more stuff open.