Required foreign keys (#2003)

* Add `required: true` option to foreign column

* Fixes NoMethodError

```
> Favourite.new.valid?
NoMethodError: undefined method `reblog?' for nil:NilClass
```

app/models/subscription.rb

@@ -4,7 +4,7 @@ class Subscription < ApplicationRecord
   MIN_EXPIRATION = 3600 * 24 * 7
   MAX_EXPIRATION = 3600 * 24 * 30
 
-  belongs_to :account
+  belongs_to :account, required: true
 
   validates :callback_url, presence: true
   validates :callback_url, uniqueness: { scope: :account_id }