Add placeholder

- Shows up as e.g. 'Building name (if any)'

Closes #79

app/src/frontend/building-edit.js

@@ -184,6 +184,7 @@ const TextInput = (props) => (
             id={props.slug} name={props.slug}
             value={props.value || ""}
             disabled={props.disabled}
+            placeholder={props.placeholder}
             onChange={props.handleChange}
             />
     </Fragment>