Add presence validation to Import (#1928)

```
*An* `ActiveRecord::StatementInvalid` *occurred while* `POST </settings/import>` *was processed by* `imports#create`
Exception
----------------
PG::NotNullViolation: ERROR:  null value in column "type" violates not-null constraint
```

app/models/import.rb

 # frozen_string_literal: true
 
 class Import < ApplicationRecord
+  FILE_TYPES = ['text/plain', 'text/csv'].freeze
+
   self.inheritance_column = false
 
-  enum type: [:following, :blocking, :muting]
+  belongs_to :account, required: true
 
-  belongs_to :account
+  enum type: [:following, :blocking, :muting]
 
-  FILE_TYPES = ['text/plain', 'text/csv'].freeze
+  validates :type, presence: true
 
   has_attached_file :data, url: '/system/:hash.:extension', hash_secret: ENV['PAPERCLIP_SECRET']
   validates_attachment_content_type :data, content_type: FILE_TYPES