Hash passwords when creating accounts

TL;DR:
- introduce the Cryptonite library
- Remove the redundant language extensions, imports, deps from Persistent
- Prefer NoContent return type for POST /accounts
- Define custom {To,From}JSON instances for Role

shell.nix

@@ -8,9 +8,7 @@ in pkgs.mkShell {
       hpkgs.resource-pool
       hpkgs.sqlite-simple
       hpkgs.warp
-      hpkgs.persistent
-      hpkgs.persistent-sqlite
-      hpkgs.persistent-template
+      hpkgs.cryptonite
     ]))
   ];
 }