Add support to use IPFS gateway to read ipfs data

This makes caching of IPFS documents easier. Browsers should cache those
by default

lib/utils/ipfs.js

@@ -1,13 +1,14 @@
 const ipfsClient = require('ipfs-http-client');
 const multihashes = require('multihashes');
+const fetch = require('node-fetch');
 
 class IPFS {
   constructor (config) {
     if (!config) {
       config = { host: 'localhost', port: '5001', protocol: 'http' };
     }
-    this._ipfsAPI = ipfsClient(config);
     this._config = config;
+    this._ipfsAPI = ipfsClient(config);
   }
 
   catAndMerge (data, deserialize) {
@@ -38,7 +39,11 @@ class IPFS {
     if (hashData.hasOwnProperty('hashSize')) {
       ipfsHash = this.encodeHash(hashData);
     }
-    return this._ipfsAPI.cat(ipfsHash);
+    if (this._config['gatewayUrl']) {
+      return fetch(`${this._config['gatewayUrl']}/${ipfsHash}`).then(r => r.text());
+    } else {
+      return this._ipfsAPI.cat(ipfsHash);
+    }
   }
 
   pin (hashData) {