Check required network before using user provider

closes #82

app/services/kredits.js

@@ -6,7 +6,7 @@ import Service from '@ember/service';
 import EmberObject from '@ember/object';
 import { computed } from '@ember/object';
 import { alias, notEmpty } from '@ember/object/computed';
-import { isEmpty } from '@ember/utils';
+import { isEmpty, isPresent } from '@ember/utils';
 
 import groupBy from 'kredits-web/utils/group-by';
 import formatKredits from 'kredits-web/utils/format-kredits';
@@ -99,10 +99,16 @@ export default Service.extend({
       }
 
       async function instantiateWithAccount (web3Provider, context) {
-        // TODO check if network is rinkeby
         console.debug('[kredits] Using user-provided instance, e.g. from Mist browser or Metamask');
         ethProvider = new ethers.providers.Web3Provider(web3Provider);
-        // const network = await ethProvider.getNetwork();
+
+        const network = await ethProvider.getNetwork();
+        if (isPresent(config.web3RequiredNetwork) &&
+            network.name !== config.web3RequiredNetwork) {
+          window.alert(`Please switch your Ethereum wallet to the "${config.web3RequiredNetwork}" network before connecting your account.`);
+          return instantiateWithoutAccount();
+        }
+
         ethProvider.listAccounts().then(accounts => {
           context.set('currentUserAccounts', accounts);
           const ethSigner = accounts.length === 0 ? null : ethProvider.getSigner();