[transaction confirmation] Show alert on confirmation when there is already a pending transaction

#Description
As a user, I want to be alerted if I an submitting a new transaction but already have a transaction pending and be prompted to to speed up existing transaction, so my new transaction is not stalled

#Situation
GIVEN: I am confirming a transaction (transaction confirmation, approve, etc)
WHEN: When I already have a transaction pending
THEN: I should get an alert notifying me of a pending transaction and prompting me to speed up existing transaction

[Error use cases](https://docs.google.com/spreadsheets/d/195Vw7sJcAeIzJFrpH1mE7MNVtb8tyPTJXADUvv4GQ3A/edit#gid=0)

Start by locating the transaction confirmation flow for confirmations and approvals, then trace how pending transactions are detected. Done means showing an alert when a transaction is already pending and prompting the user to speed up the existing transaction rather than leaving the new transaction stalled.

Written by the indexing model from the issue text.