Keep netId in browser's storage and use it when page first loading if possible

src/index.js

@@ -102,6 +102,7 @@ class AppMainRouter extends Component {
 
   onNetworkChange = e => {
     commonStore.showLoading(getNetworkBranch(e.value))
+    window.localStorage.netId = e.value
     window.sessionStorage.netId = e.value
     contractsStore.resetContracts()
     ballotsStore.reset()

src/utils/getWeb3.js

@@ -23,14 +23,26 @@ export default async function getWeb3(netId, onAccountChange) {
   }
 
   if (!netId) {
+    // Load for the first time in the current browser's session
     if (web3) {
+      // MetaMask (or another plugin) is injected
       netId = await web3.eth.net.getId()
       if (!(netId in constants.NETWORKS)) {
-        netId = defaultNetId
+        // If plugin's netId is unsupported, try to use
+        // the previously chosen netId
+        netId = window.localStorage.netId
       }
     } else {
+      // MetaMask (or another plugin) is not injected,
+      // so try to use the previously chosen netId
+      netId = window.localStorage.netId
+    }
+    if (!(netId in constants.NETWORKS)) {
+      // If plugin's netId and/or previously chosen netId are not supported,
+      // fallback to default netId
       netId = defaultNetId
     }
+    window.localStorage.netId = netId
     window.sessionStorage.netId = netId
   }