Use string for class_name option for lazy autoload class

Passing constant into this option is deprecated in Rails 5.2
https://github.com/rails/rails/commit/8312a0d22212798864f142b5a94805e0baa6c562

app/models/forked_project_link.rb

 class ForkedProjectLink < ActiveRecord::Base
-  belongs_to :forked_to_project, class_name: Project
-  belongs_to :forked_from_project, class_name: Project
+  belongs_to :forked_to_project, class_name: 'Project'
+  belongs_to :forked_from_project, class_name: 'Project'
 end

app/models/project.rb

@@ -122,7 +122,7 @@ class Project < ActiveRecord::Base
   # Merge Requests for target project should be removed with it
   has_many :merge_requests,     dependent: :destroy, foreign_key: 'target_project_id'
   # Merge requests from source project should be kept when source project was removed
-  has_many :fork_merge_requests, foreign_key: 'source_project_id', class_name: MergeRequest
+  has_many :fork_merge_requests, foreign_key: 'source_project_id', class_name: 'MergeRequest'
   has_many :issues,             dependent: :destroy
   has_many :labels,             dependent: :destroy, class_name: 'ProjectLabel'
   has_many :services,           dependent: :destroy