userWidget: Fix the padding in RTL

https://bugzilla.gnome.org/show_bug.cgi?id=712638
authPrompt: Explicitly set horizontal alignment

When set to fill, the label will always end up left-aligned, which
is only correct in LTR locales. Set the alignment explicitly to
work in both RTL and LTR locales.

https://bugzilla.gnome.org/show_bug.cgi?id=712638
unlockDialog: Explicitly set horizontal alignment

When set to fill, the label will always end up left-aligned, which
is only correct in LTR locales. Set the alignment explicitly to
work in both RTL and LTR locales.

https://bugzilla.gnome.org/show_bug.cgi?id=712638