fix: Blockscout and Etherscan lookups are off until the user turns them on (closes #668)

Out of the box the wallet sent the user's addresses to Blockscout for history
and token lookups, and each recipient to Etherscan for its label. Two settings,
"Use Blockscout" and "Check recipients on Etherscan", now gate those requests
where they are made. Both are off in a new profile and offered unticked on
Welcome and in Settings. While one is off, the screens that relied on it say so
in a plain line naming the setting; after either changes in Settings, Back
draws the screen it lands on again. README and the user guide say what is
contacted by default and what each setting adds.

Model: opus-5-5

src/shared/persistedState.js

@@ -48,6 +48,12 @@ const DEFAULT_STATE = {
     hideDustTransactions: true,
     dustThresholdGwei: 100000,
     utcTimestamps: false,
+    // "Use Blockscout" and "Check recipients on Etherscan" in Settings, one
+    // setting each for every network. Both off until the user turns them on:
+    // the requests they allow send the user's addresses to a service other
+    // than the RPC endpoint.
+    useBlockscout: false,
+    checkRecipientsOnEtherscan: false,
     fraudContracts: [],
     tokenHolderCache: {},
     theme: "system",
@@ -406,6 +412,12 @@ function normalizePersisted(saved) {
             : 100000;
     out.utcTimestamps =
         saved.utcTimestamps !== undefined ? saved.utcTimestamps : false;
+    out.useBlockscout =
+        saved.useBlockscout !== undefined ? saved.useBlockscout : false;
+    out.checkRecipientsOnEtherscan =
+        saved.checkRecipientsOnEtherscan !== undefined
+            ? saved.checkRecipientsOnEtherscan
+            : false;
     // A list of contract addresses, floored the same way: send.js builds its
     // fraud set as `(state.fraudContracts || []).map((a) => a.toLowerCase())`
     // and filterTransactions() maps the same list through normalizeAddress(),