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/Models/Traits/NoTimestamp.js

@@ -0,0 +1,15 @@
+'use strict'
+
+class NoTimestamp {
+  register (Model) {
+    Object.defineProperties(Model, 'createdAtColumn', {
+      get () { return null }
+    })
+
+    Object.defineProperties(Model, 'updatedAtColumn', {
+      get () { return null }
+    })
+  }
+}
+
+module.exports = NoTimestamp