Add time estimates to pending transactions

[danjm]

As shown in the below gif, you can see estimates of how long a transaction will take in the advanced tab of the gas edit modal when accessed from the confirm screen.

This task is to add similar time estimates to "pending" transactions in the transaction list. In the above gif, you can see the transaction in the pending state in the list, after it is submitted. The estimate of the remaining time before the transaction is confirmed should be shown in the space between the "pending" label and the "- 0ETH"

A design can be seen here: MetaMask/Design#25 (comment)

If possible, make it so that the time updates in real time, perhaps at intervals. Use your judgement to optimize UI.