disables the fee slider in the swap dialog for reverse swaps as the tx
fee for claiming is not configurable by the user. Also replaces calls to
`sm.get_swap_tx_fee()` with `sm.get_fee_for_txbatcher()` as this is the
correct fee estimate for claim transactions, instead of the config fee
eta used by `get_swap_tx_fee()`.
37181cd3a8