Use ethers.js instead of the old web3

the web3 we are using is old and everything will change with web3 1.0
which is currently in beta for ages.
ethers.js seems to be a bit more lightweight and implements pretty much
the same API so let's give it a try.

app/components/add-proposal/component.js

@@ -18,7 +18,8 @@ export default Component.extend({
   inProgress: false,
 
   isValidRecipient: computed('proposal.recipientAddress', function() {
-    return this.get('kredits.web3').isAddress(this.get('proposal.recipientAddress'));
+    // TODO: add proper address validation
+    return this.get('proposal.recipientAddress') !== ''
   }),
 
   isValidAmount: computed('proposal.amount', function() {