Let admins add users to three groups on zhwikivoyage

This patch makes it possible for users in the 'sysop'
user group on zhwikivoyage to add and remove other
users from the 'confirmed', 'import' and 'transwiki'
user groups.

Bug: 60085
Change-Id: I3e647702acdf38719d959a7acf18b30670fa2a59