TUX refactoring for performance

ApiQueryMessageGroups: root parameter can now be used together with
format=flat and then it only returns that group.

JavaScript changes:

mw.translate.loadMessageGroups has been removed.

mw.translate.getMessageGroup now takes a second parameter for props to load.

mw.translate.getGroup has been renamed to mw.translate.findGroup and the
second parameter is no longer optional. It is also now in the base module
ext.translate.base.

Module ext.translate.groupselector was refactored to load group data
itself to improve performance. It's still loading the priority*
properties which are among the slowest properties to load currently.

Module ext.translate.special.translate was refactored to load data only
for the currently selected group to improve performance.

https://mingle.corp.wikimedia.org/projects/internationalization/cards/3854

Bug: 53748
Change-Id: I2f83cd0666386745820b0fd665c2cba703bbd560