feat: add NoTimestamp trait + ConvertEmptyStringsToNull middleware (#30)

Hey! 👋 

This PR add some middleware and traits by default.

**Middleware**:

- `AllowGuestOnly` throws an exception if you are authenticated.
- `ConvertEmptyStringsToNull` converts empty strings in the request body to `null`

**Trait**:

- `NoTimestamp` removes the need for `created_at` and `updated_at` fields.
- `FormatDate` formats the date following a convention.

**FormatDate**

The convention is:
- Ends with `_at` => DateTime
- Ends with `_on` => Date

You can change the format you use in your UI with two getters.

```js
class MyModel extends Model {
  static get formatDate () { return 'DD.MM.YYYY' }
  static get formatDateTime () { return 'DD.MM.YYYY HH:mm' }
}
```

You can also directly change default values inside the Trait since it is a user Trait and not defined in the core.

<hr>

If this is merged, I'll provide the same PR to the `API` blueprint.

app/Middleware/ConvertEmptyStringsToNull.js

@@ -0,0 +1,17 @@
+'use strict'
+
+class ConvertEmptyStringsToNull {
+  async handle ({ request }, next) {
+    if (Object.keys(request.body).length) {
+      request.body = Object.assign(
+        ...Object.keys(request.body).map(key => ({
+          [key]: request.body[key] !== '' ? request.body[key] : null
+        }))
+      )
+    }
+
+    await next()
+  }
+}
+
+module.exports = ConvertEmptyStringsToNull