* Only display web push notifications after API call (fixes #7902)

* Decode then truncate instead of truncating then decoding in webpush serializer

> Good lord what is happening in there

Previously the contents of the Web Push API payloads closely resembled the structure of JavaScript's [Notification](https://developer.mozilla.org/en-US/docs/Web/API/Notification). But now that the API is open to non-browser apps, and given that there is no required coupling between contents of the payload and a Notification object, here is how I changed the payload:

```json
{ 
  "access_token": "...",
  "preferred_locale": "en",
  "notification_id": "12345",
  "notification_type": "follow",
  "title": "So and so followed you",
  "body": "This is my bio",
  "icon": "https://example.com/avatar.png"
}
```

The title, body and icon attributes are included as a fallback so you can construct a minimal notification if you cannot perform a network request to the API to get more data.

- POST /api/v1/push/subscription
- PUT /api/v1/push/subscription
- DELETE /api/v1/push/subscription
- New OAuth scope: "push" (required for the above methods)

* Fix NoMethodError in Web::PushSubscription

```
undefined method `site_contact_email' for #<Class:0x00005976d13c40>

/usr/local/bundle/gems/activerecord-5.1.3/lib/active_record/dynamic_matchers.rb:22:in `method_missing'
/usr/local/bundle/gems/attr_encrypted-3.0.3/lib/attr_encrypted.rb:295:in `method_missing'
/usr/local/bundle/gems/attr_encrypted-3.0.3/lib/attr_encrypted/adapters/active_record.rb:129:in `method_missing_with_attr_encrypted'
/mastodon/app/models/web/push_subscription.rb:53:in `push_payload'
```

* Specify serializer in Web::NotificationSerializer

* Refactor Web::PushSubscription, remove welcome message

* Add missing helper

* Use locale of the receiver on push notifications (#4519)

* Remove unused translations

* Fix dir on notifications