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. README and the user guide say what is
contacted by default and what each setting adds. Unit tests show each request
skipped while its setting is off and made while it is on; the Chrome suite
watches every request from a fresh install, then turns each setting on.

Model: opus-5-5

tests/persistedFieldContract.test.js

@@ -618,6 +618,8 @@ const CONTRACT = [
         "hideFraudContracts",
         "hideDustTransactions",
         "utcTimestamps",
+        "useBlockscout",
+        "checkRecipientsOnEtherscan",
         "debugMode",
     ].map((field) => ({
         field,
@@ -625,8 +627,9 @@ const CONTRACT = [
         // A flag: only ever tested for truthiness, and written back verbatim.
         hostile: [42, "notabool", { a: 1 }],
         // Both answers to that truthiness test have to be driven, and 0 is a
-        // value src/ never writes for a flag. For utcTimestamps and debugMode
-        // the falsy answer is also the DEFAULT_STATE default.
+        // value src/ never writes for a flag. For utcTimestamps,
+        // useBlockscout, checkRecipientsOnEtherscan and debugMode the falsy
+        // answer is also the DEFAULT_STATE default.
         falsy: [0],
     })),
 ];