[RN] Cache configurations in localStorage

This only helps iff there is a short transient network error which prevents the
configuration from being loaded. In such case, use the cached version in
localStorage, which may not match the shard, but it's (probably!) better than
nothing.

In case there is no Internet connectivity, an error will be produced as soon as
the XMPP connection is attempted anyway.

react/features/app/actions.js

@@ -186,12 +186,49 @@ function _loadConfig({ contextRoot, host, protocol, room }) {
 
     // TDOO userinfo
 
-    let url = `${protocol}//${host}${contextRoot || '/'}config.js`;
+    const rootUrl = `${protocol}//${host}${contextRoot || '/'}`;
+    let url = `${rootUrl}config.js`;
 
     // XXX In order to support multiple shards, tell the room to the deployment.
     room && (url += `?room=${room.toLowerCase()}`);
 
     /* eslint-enable no-param-reassign */
 
-    return loadConfig(url);
+    const key = `config/${rootUrl}`;
+
+    return (
+        loadConfig(url)
+            .then(config => {
+                // Try to store the configuration in localStorage. If the
+                // deployment specified the 'getroom' option as a function, for
+                // example, we cannot store it, so don't.
+                try {
+                    window.localStorage.setItem(key, JSON.stringify(config));
+                } catch (e) {
+
+                    // Ignore the error, we won't cache this config.
+                }
+
+                return config;
+            })
+            .catch(error => {
+                // We failed to load the requested config, try to use the last
+                // one which was fetched for that deployment. It may not match
+                // the shard, but it's probably better than nothing.
+                const config = window.localStorage.getItem(key);
+
+                if (config) {
+                    try {
+                        return JSON.parse(config);
+                    } catch (e) {
+
+                        // Somehow incorrect data ended up in the storage. Clean
+                        // up.
+                        window.localStorage.removeItem(key);
+                    }
+                }
+
+                throw error;
+            })
+    );
 }