Specify languages for MessageGroupStats and groups for LanguageStats

API input to select list of languages to display on
Special:MessageGroupStats using language codes.
eg &languages=en,fr
Select list of groups to display on Special:LanguageStats.
e.g. &groups=gp1,gp2

Bug: 57514
Change-Id: Ifab06704223685d6490d9be7bd62281259eb4813
Merge "Specify languages for MessageGroupStats and groups for LanguageStats"