Make the config optional

Accept 404 errors from getting the config and start MatrixChat with no config, make other errors display a simple error message to prevent a completely blank page if the config does fail to load.

vector/config.json

@@ -1 +0,0 @@
-../config.json

vector/config.sample.json

@@ -0,0 +1,7 @@
+{
+    "default_hs_url": "https://matrix.org",
+    "default_is_url": "https://vector.im",
+    "brand": "Vector",
+    "integrations_ui_url": "http://localhost:8081/",
+    "integrations_rest_url": "http://localhost:5050"
+}