refactor: move the calldata decoder to src/shared/calldataDecoder.js (closes #759)

decodeCalldata() and tokenAmountText() move out of the TxApproval view into
src/shared/calldataDecoder.js. TxApproval, TransactionDetail and the decoder
tests import them from there.

tokenSourcesByChainId() moves from the popup view helpers to
src/shared/state.js, beside currentNetwork(), so the decoder requires nothing
under src/popup/. It reads only the state singleton and the network table.

Model: opus-5-5

src/popup/views/send.js

@@ -10,9 +10,13 @@ const {
     renderAddressHtml,
     attachCopyHandlers,
     goBack,
-    tokenSourcesByChainId,
 } = require("./helpers");
-const { state, currentAddress, currentNetwork } = require("../../shared/state");
+const {
+    state,
+    currentAddress,
+    currentNetwork,
+    tokenSourcesByChainId,
+} = require("../../shared/state");
 let ctx;
 const { getProvider } = require("../../shared/balances");
 const { resolveTokenDecimals } = require("../../shared/approvalAmount");