Move chat integrations to `Integrations::`

.rubocop_manual_todo.yml

app/models/chat_team.rb

app/models/concerns/notification_branch_selection.rb

app/models/integrations/base_chat_notification.rb

app/models/integrations/chat_message/base_message.rb

app/models/integrations/chat_message/pipeline_message.rb

app/models/integrations/chat_message/push_message.rb

app/models/integrations/discord.rb

app/models/integrations/hangouts_chat.rb

app/models/integrations/mattermost.rb

app/models/integrations/microsoft_teams.rb

app/models/integrations/slack.rb

app/models/integrations/unify_circuit.rb

app/models/integrations/webex_teams.rb

app/models/project.rb

app/models/project_services/chat_notification_service.rb

app/models/project_services/discord_service.rb

app/models/project_services/hangouts_chat_service.rb

app/models/project_services/mattermost_service.rb

...els/project_services/mattermost_slash_commands_service.rb