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

tests/holders.test.js

@@ -150,19 +150,23 @@ describe("the balance-list spam gate", () => {
 
     test("a token with plenty of reported holders is listed", async () => {
         respondWith([item()]);
-        const balances = await fetchTokenBalances(HOLDER, BLOCKSCOUT, []);
+        const balances = await fetchTokenBalances(HOLDER, BLOCKSCOUT, true, []);
         expect(balances).toHaveLength(1);
         expect(balances[0].holders).toBe(50000);
     });
 
     test("a token reporting zero holders is still excluded", async () => {
         respondWith([item({ token: { holders_count: "0" } })]);
-        expect(await fetchTokenBalances(HOLDER, BLOCKSCOUT, [])).toEqual([]);
+        expect(await fetchTokenBalances(HOLDER, BLOCKSCOUT, true, [])).toEqual(
+            [],
+        );
     });
 
     test("an unknown holder count does not admit an unvouched token", async () => {
         respondWith([item({ token: { holders_count: null } })]);
-        expect(await fetchTokenBalances(HOLDER, BLOCKSCOUT, [])).toEqual([]);
+        expect(await fetchTokenBalances(HOLDER, BLOCKSCOUT, true, [])).toEqual(
+            [],
+        );
     });
 
     // The path that reaches the send selector and the history filter: a token
@@ -171,7 +175,7 @@ describe("the balance-list spam gate", () => {
     // zero that would then hide it downstream.
     test("a tracked token with an unknown count is listed with holders null", async () => {
         respondWith([item({ token: { holders_count: undefined } })]);
-        const balances = await fetchTokenBalances(HOLDER, BLOCKSCOUT, [
+        const balances = await fetchTokenBalances(HOLDER, BLOCKSCOUT, true, [
             { address: NOVEL_TOKEN.toUpperCase() },
         ]);
         expect(balances).toHaveLength(1);
@@ -188,7 +192,7 @@ describe("the balance-list spam gate", () => {
                 },
             }),
         ]);
-        const balances = await fetchTokenBalances(HOLDER, BLOCKSCOUT, []);
+        const balances = await fetchTokenBalances(HOLDER, BLOCKSCOUT, true, []);
         expect(balances).toHaveLength(1);
         expect(balances[0].holders).toBeNull();
     });