Fix and improve gas estimation in new-ui send screen

[danjm]

We need to fix and then improve gas estimation in the new-ui send screen.

Requirements include:

• use the same algorithm for calculating gas as is used in old-ui, be sure to pass same data (when available) to estimateGas
• on component mount, we need to make an initial estimate without a to address
• when the to address changes, we need to update the estimate
• prevent the user from customizing the limit to a value below the minimum required given the estimate
• if the user customizes the gas limit and then changes the to address, only change the gas limit if it is below the minimum required according to the new estimate
• Handle the special case of estimating for token transfers.
  • correct value in the to address passed to estimateGas
  • (?) update 'data' when 'To' or amount fields in send form changes (?)
• Nice to have: update the estimate if blockGasLimit updates
• Nice to have: give user an indication that the gas limit has been updated