Add #resource_parent to Snippet

In a future commit we will be calling #resource_parent on an object we
don't know the exact type of. But all of them respond to that method,
except for Snippet. With this change we can safely call it without
getting a NoMethodError.

app/models/snippet.rb

@@ -40,6 +40,7 @@ class Snippet < ApplicationRecord
 
   belongs_to :author, class_name: 'User'
   belongs_to :project
+  alias_method :resource_parent, :project
 
   has_many :notes, as: :noteable, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_many :user_mentions, class_name: "SnippetUserMention", dependent: :delete_all # rubocop:disable Cop/ActiveRecordDependent