fix(ui): increase currency selector z-index

Ensure that the currency selector dropdown on the pay/request form shows
ontop of other elements on the page to ensure that the full area of the
dropdown is clickable.

app/components/Form/Pay.scss

@@ -123,6 +123,7 @@
       visibility: hidden;
       position: absolute;
       top: 30px;
+      z-index: 10;
 
       &.active {
         visibility: visible;

app/components/Form/Request.scss

@@ -102,6 +102,7 @@
       visibility: hidden;
       position: absolute;
       top: 30px;
+      z-index: 10;
 
       &.active {
         visibility: visible;