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 and each recipient's address to Etherscan. Two settings, "Use Blockscout" and "Check recipients on Etherscan", now gate every request that sends the user's data to those services. Both start off and are offered unticked on Welcome and in Settings. Each request reads its setting from storage before it asks. An answer that lands after the setting went off is dropped. While a setting is off, the screens that rely on it say so. ENS lookups ask only the RPC endpoint, so a name resolved off-chain does not resolve.

RULES.md's External Communication item is now out of date. Its new wording will be proposed on #211 once this lands.

Model: opus-5-5 (implementation, summary); fable-5-1 (prose)

src/shared/chainSwitch.js

@@ -4,7 +4,7 @@
 //
 // - onChainSwitch() below makes the switch the user picks in Settings, with
 //   applyChainSwitchFields() (chainSwitchFields.js).
-// - followNetworkSwitch() in src/popup/index.js takes a switch the background
+// - followStoredSettings() in src/popup/index.js takes a switch the background
 //   or another popup page saved, from the stored record, with
 //   takeStoredNetwork() (state.js), which copies it with
 //   copyChainSwitchFields() (chainSwitchFields.js).
@@ -45,7 +45,7 @@ function clearPopupCaches() {
 //
 // `state` is switched inside the save that stores the switch, holding the
 // lock, so it is never switched while a follow of a switch saved elsewhere
-// waits for the lock (followNetworkSwitch() in src/popup/index.js): that
+// waits for the lock (followStoredSettings() in src/popup/index.js): that
 // follow runs first, and this switch is made after it.
 async function onChainSwitch(newNetworkId) {
     const { state, saveState } = require("./state");