Introduce maximum session time for terminal websocket connection

app/controllers/admin/application_settings_controller.rb

app/models/application_setting.rb

app/models/project_services/kubernetes_service.rb

app/views/admin/application_settings/_form.html.haml

changelogs/unreleased/terminal-max-session-time.yml

..._add_terminal_max_session_time_to_application_settings.rb

db/schema.rb

doc/administration/integration/terminal.md

doc/api/settings.md

lib/api/entities.rb

lib/api/settings.rb

lib/gitlab/kubernetes.rb

lib/gitlab/workhorse.rb

spec/lib/gitlab/workhorse_spec.rb

spec/models/project_services/kubernetes_service_spec.rb

spec/support/kubernetes_helpers.rb