Add code suggestions to namespace settings

What does this MR do and why?

Adds code_suggestions boolean field to namespace settings table in order to allow groups/namespaces to control their using of the feature as per #405126

Migration output

❯ be rails db:migrate
main: == [advisory_lock_connection] object_id: 274240, pg_backend_pid: 53290
main: == 20230407164616 AddCodeSuggestionsToNamespaceSettings: migrating ============
main: -- add_column(:namespace_settings, :code_suggestions, :boolean, {:default=>false, :null=>false})
main:    -> 0.0029s
main: == 20230407164616 AddCodeSuggestionsToNamespaceSettings: migrated (0.0068s) ===

main: == [advisory_lock_connection] object_id: 274240, pg_backend_pid: 53290
ci: == [advisory_lock_connection] object_id: 274540, pg_backend_pid: 53295
ci: == [advisory_lock_connection] object_id: 274540, pg_backend_pid: 53295
❯ be rails db:migrate:down:main VERSION=20230407164616
main: == [advisory_lock_connection] object_id: 274000, pg_backend_pid: 53973
main: == 20230407164616 AddCodeSuggestionsToNamespaceSettings: reverting ============
main: -- remove_column(:namespace_settings, :code_suggestions, :boolean, {:default=>false, :null=>false})
main:    -> 0.0031s
main: == 20230407164616 AddCodeSuggestionsToNamespaceSettings: reverted (0.0121s) ===

main: == [advisory_lock_connection] object_id: 274000, pg_backend_pid: 53973

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #405126