Register: require at least 20-byte passwords.

src/requests/register.cr

@@ -34,8 +34,8 @@ class AuthD::Request
 			end
 
 			# In this case we should not accept its registration.
-			if @password.size < 4
-				return Response::Error.new "password too short"
+			if @password.size < 20
+				return Response::Error.new "password too short (< 20 characters)"
 			end
 
 			uid = authd.new_uid