Enable frozen string in rest of app/models/**/*.rb

Partially addresses #47424.

app/models/programming_language.rb

+# frozen_string_literal: true
+
 class ProgrammingLanguage < ActiveRecord::Base
   validates :name, presence: true
   validates :color, allow_blank: false, color: true

app/models/project_services/asana_service.rb

+# frozen_string_literal: true
+
 require 'asana'
 class AsanaService < Service

app/models/project_services/assembla_service.rb

+# frozen_string_literal: true
+
 class AssemblaService < Service
   prop_accessor :token, :subdomain
   validates :token, presence: true, if: :activated?

app/models/project_services/bamboo_service.rb

+# frozen_string_literal: true
+
 class BambooService < CiService
   include ReactiveService

app/models/project_services/bugzilla_service.rb

+# frozen_string_literal: true
+
 class BugzillaService < IssueTrackerService
   validates :project_url, :issues_url, :new_issue_url, presence: true, public_url: true, if: :activated?

app/models/project_services/buildkite_service.rb

+# frozen_string_literal: true
+
 require "addressable/uri"
 class BuildkiteService < CiService

app/models/project_services/builds_email_service.rb

+# frozen_string_literal: true
+
 # This class is to be removed with 9.1
 # We should also by then remove BuildsEmailService from database
 class BuildsEmailService < Service

app/models/project_services/campfire_service.rb

+# frozen_string_literal: true
+
 class CampfireService < Service
   prop_accessor :token, :subdomain, :room
   validates :token, presence: true, if: :activated?
@@ -82,7 +84,7 @@ class CampfireService < Service
     before = push[:before]
     after = push[:after]
-    message = ""
+    message = []
     message << "[#{project.full_name}] "
     message << "#{push[:user_name]} "
@@ -95,6 +97,6 @@ class CampfireService < Service
       message << "#{project.web_url}/compare/#{before}...#{after}"
     end
-    message
+    message.join
   end
 end

app/models/project_services/chat_message/base_message.rb

+# frozen_string_literal: true
+
 require 'slack-notifier'
 module ChatMessage

app/models/project_services/chat_message/issue_message.rb

+# frozen_string_literal: true
+
 module ChatMessage
   class IssueMessage < BaseMessage
     attr_reader :title

app/models/project_services/chat_message/merge_message.rb

+# frozen_string_literal: true
+
 module ChatMessage
   class MergeMessage < BaseMessage
     attr_reader :merge_request_iid

app/models/project_services/chat_message/note_message.rb

+# frozen_string_literal: true
+
 module ChatMessage
   class NoteMessage < BaseMessage
     attr_reader :note

app/models/project_services/chat_message/pipeline_message.rb

+# frozen_string_literal: true
+
 module ChatMessage
   class PipelineMessage < BaseMessage
     attr_reader :ref_type

app/models/project_services/chat_message/push_message.rb

+# frozen_string_literal: true
+
 module ChatMessage
   class PushMessage < BaseMessage
     attr_reader :after

app/models/project_services/chat_message/wiki_page_message.rb

+# frozen_string_literal: true
+
 module ChatMessage
   class WikiPageMessage < BaseMessage
     attr_reader :title

app/models/project_services/chat_notification_service.rb

+# frozen_string_literal: true
+
 # Base class for Chat notifications services
 # This class is not meant to be used directly, but only to inherit from.
 class ChatNotificationService < Service

app/models/project_services/ci_service.rb

+# frozen_string_literal: true
+
 # Base class for CI services
 # List methods you need to implement to get your CI service
 # working with GitLab Merge Requests

app/models/project_services/custom_issue_tracker_service.rb

+# frozen_string_literal: true
+
 class CustomIssueTrackerService < IssueTrackerService
   validates :project_url, :issues_url, :new_issue_url, presence: true, public_url: true, if: :activated?

app/models/project_services/deployment_service.rb

+# frozen_string_literal: true
+
 # Base class for deployment services
 #
 # These services integrate with a deployment solution like Kubernetes/OpenShift,

app/models/project_services/drone_ci_service.rb

+# frozen_string_literal: true
+
 class DroneCiService < CiService
   include ReactiveService