Stop upper-casing exported private key

We were showing exported private keys in all upper case. This is
atypical, and many other wallets (e.g. MyCrypto) expect private keys
for imported accounts to be lower-cased. Our own account import is
tolerant of either case.

ui/app/css/itcss/components/modal.scss

@@ -216,7 +216,6 @@
   overflow: hidden;
   resize: none;
   padding: 9px 13px 8px;
-  text-transform: uppercase;
 }
 
 .modal-close-x::after {