Add new border scss variable to old admin variables

Tags' rules are still coming from the old admin styles hence had to add
the new (admin_v3) border variable to the old one.
Has a hard coded colour value of #2e3132 as it has no access
to the new colours.

app/webpacker/css/admin/globals/variables.scss

@@ -146,6 +146,7 @@ $h2-size:                        $h3-size + 2 !default;
 $h1-size:                        $h2-size + 2 !default;
 
 $border-radius:                  3px !default;
+$border-input:                   1px solid #2e3132; // Copied over from admin_v3 variables as a temporary solution
 
 $font-weight-bold:               600 !default;
 $font-weight-normal:             400 !default;