authPrompt: Refine entry width

The current entry has a hardcoded size of 320px,
which doesn't really fit the lock screen mockups.

Remove this hardcoded size from AuthPrompt entries
and make them horizontally expand.

data/theme/gnome-shell-sass/_common.scss

@@ -1929,6 +1929,7 @@ StScrollBar {
 
   StEntry {
     @extend %search_entry;
+    width: -1px;
     border-radius: $button_radius;
     @if $variant=='dark' {
       $_gdm_entry_bg: transparentize(lighten(desaturate(#241f31, 20%), 2%), 0.5);