Remove iron-input polymer dependency

Replace the iron-input wrapper and its bind-value attribute with a
plain HTML input element using Lit's .value property binding.

iron-input is a Polymer component that no longer renders properly
in Gerrit v3.13 as part of the ongoing migration away from Polymer.

Change-Id: Iee0a15a2d430cae9bd0c6617cce50a6213b2a3b8