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. It sent 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 install 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. Unticking "Use Blockscout"
drops every untracked token from every address. After either changes in
Settings, Back draws again any screen a reopened popup can draw. ENS lookups
ask only the RPC endpoint.

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],
     })),
 ];