chore: add JSON user config defaulting to using local Astarte instance

Add a default JSON configuration that the dashboard automatically
discovers when running it with `npm run start`, i.e. for local
development.
The configuration assumes a local Astarte instance being available, for
example by following the official "Astarte in 5 minutes" guide from
Astarte's documentation.
This change is aimed at improving the developer experience, so that the
dashboard is ready to be used out of the box with a local Astarte,
provided that the user already has a realm and an associated access
token.

Signed-off-by: Davide Briani <davide.briani@secomind.com>

Author: davidebriani

.gitignore

@@ -21,5 +21,3 @@
 npm-debug.log*
 yarn-debug.log*
 yarn-error.log*
-
-/public/user-config

README.md

@@ -150,7 +150,7 @@ Astarte Dashboard is written in TypeScript using the React framework and npm to
    ```sh
    npm install
    ```
-3. Place your configuration file in `src/user-config/config.json`
+3. Place your configuration file in `public/user-config/config.json`
 4. Start the dev server
    ```sh
    npm run start

public/user-config/config.json

@@ -0,0 +1,10 @@
+{
+    "astarte_api_url": "http://api.astarte.localhost",
+    "default_auth": "token",
+    "enable_flow_preview": false,
+    "auth": [
+        {
+            "type": "token"
+        }
+    ]
+}