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/mute.rb

@@ -3,9 +3,8 @@
 class Mute < ApplicationRecord
   include Paginable
 
-  belongs_to :account
-  belongs_to :target_account, class_name: 'Account'
+  belongs_to :account, required: true
+  belongs_to :target_account, class_name: 'Account', required: true
 
-  validates :account, :target_account, presence: true
   validates :account_id, uniqueness: { scope: :target_account_id }
 end