Auto-detect plausible tx retry attempts and warn users

When sending a new tx whose parameters are identical to a pending transaction, warn users:

> It looks like you might be re-trying an earlier transaction. Submitting this will result in an additional transaction, which can send more funds than you mean to! If you want to make your earlier transaction go through faster, *retry it with higher gas* instead. (Cancel) (Continue)

Start by locating the transaction submission flow and the handling of pending transactions. Compare a new transaction's parameters with pending transactions, then verify that matching parameters show the specified warning with Cancel and Continue choices. Done means the warning appears only for plausible retries and explains the higher-gas retry option.

Written by the indexing model from the issue text.