Cache fetched Dapps (#3804)

* Have a singleton DappsFetcher so we don't realod them at each page load

* Better dapps Fetcher : event based listener
  Update on meta change

* Remove dapps fetcher => all in singleton dapps store + utils

* Modify header to Parity

js/src/contracts/dappreg.js

@@ -22,8 +22,12 @@ export default class DappReg {
     this.getInstance();
   }
 
+  getContract () {
+    return this._registry.getContract('dappreg');
+  }
+
   getInstance () {
-    return this._registry.getContractInstance('dappreg');
+    return this.getContract().then((contract) => contract.instance);
   }
 
   count () {