Unify Reference Period settings for reports

[Peter Klein]

In reports settings there are 4 items to choose for reference period - 3 moths, 6 months, 9 months and 1 year.
In report itself it is possible to choose from last quarter (last 3 months) up to last 24 months stepping by 1 month.
In my opinion these two choices should be identical
- in the number of choices
- in the text used to display these choices (3 moths versus last quarter, 6 months versus last 6 moths)

In fact "last six month" could be "next six month" - when the reference moth is moved forward.

P.S.
text "Last 9 month" comes from key: report_last_9_months
and the rest of texts come from key: report_last_n_months
This could lead to inconsistent translations - like it is in the case of Slovak language.