Convert admin mode feature flag to application setting

app/controllers/application_controller.rb

app/controllers/concerns/enforces_admin_authentication.rb

app/controllers/concerns/sessionless_authentication.rb

app/controllers/ldap/omniauth_callbacks_controller.rb

app/controllers/omniauth_callbacks_controller.rb

app/helpers/application_settings_helper.rb

app/helpers/nav_helper.rb

app/models/application_setting.rb

app/models/application_setting_implementation.rb

app/policies/base_policy.rb

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

app/views/layouts/nav/_dashboard.html.haml

...d/refactor-convert-admin-mode-feature-flag-to-setting.yml

config/feature_flags/development/user_mode_in_session.yml

...e/20210309160106_add_admin_mode_to_application_setting.rb

db/schema_migrations/20210309160106

db/structure.sql

doc/api/settings.md

doc/development/secure_coding_guidelines.md

doc/user/admin_area/settings/sign_in_restrictions.md