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. It sent each
recipient to Etherscan for its label. Two settings, "Use Blockscout" and "Check
recipients on Etherscan", now gate those requests. 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. Each gated request reads its setting
from storage before it asks and when the answer lands. An answer that lands
after the setting went off is dropped. An open popup takes a change another page
saved and draws its screens again. While "Use Blockscout" is off, no address
keeps an untracked token. ENS lookups ask only the RPC endpoint.

Model: opus-5-5

tests/sendMax.test.js

@@ -100,7 +100,10 @@ global.fetch = jest.fn(() => {
 });
 
 const { makeStorageStub } = require("./support/storageStub");
-global.chrome = { storage: makeStorageStub(), runtime: { sendMessage() {} } };
+global.chrome = {
+    storage: makeStorageStub({ autistmask: { useBlockscout: true } }),
+    runtime: { sendMessage() {} },
+};
 
 // A stub DOM: every id resolves to a recording element.
 const elements = new Map();
@@ -213,6 +216,9 @@ async function refreshWith(balanceWei, tokenItems = []) {
     ];
     state.selectedWallet = 0;
     state.selectedAddress = 0;
+    // Blockscout is asked only while the stored "Use Blockscout" is on. The
+    // screens save this page's copy of it over the stored one.
+    state.useBlockscout = true;
     await refreshBalances(
         state.wallets,
         "https://rpc.example.invalid",