Set 'skin' template parameter rather than override parent method

BaseTemplate's getSkin() method is perfectly adequate if you simply set a
skin value where it expects it. Doing so obviates the need to reimplement
getSkin() in UserloginTemplateVForm and UsercreateTemplateVForm.

Backported from master for new opt-in Create account form.

Change-Id: Icd22de2218fc96cb67c11d0c1763b607743fe3b6
(cherry picked from commit 8408295b72286f8837e2b462786cc14a3e9a3b00)
Merge "Set 'skin' template parameter rather than override parent method" into wmf/1.22wmf2