Add exclude from free user cap to namespace settings

What does this MR do and why?

• adds the ability to exclude certain free namespaces from the free user cap see overall goals in https://gitlab.com/groups/gitlab-org/-/epics/8041
• adds first use of this setting in the preview area

Subsequent issues/mr's will add the ability to change this setting programatically.

Related to #362097

Related to #362098

DB migration output

Click to expand

❯ be rails db:migrate
== 20220511151646 AddExcludeFromFreeUserCapToNamespaceSettings: migrating =====
-- transaction_open?()
   -> 0.0000s
-- add_column(:namespace_settings, :exclude_from_free_user_cap, :boolean, {:null=>false, :default=>false})
   -> 0.0106s
== 20220511151646 AddExcludeFromFreeUserCapToNamespaceSettings: migrated (0.0251s)

== 20220511151646 AddExcludeFromFreeUserCapToNamespaceSettings: migrating =====
-- transaction_open?()
   -> 0.0000s
-- add_column(:namespace_settings, :exclude_from_free_user_cap, :boolean, {:null=>false, :default=>false})
   -> 0.0105s
== 20220511151646 AddExcludeFromFreeUserCapToNamespaceSettings: migrated (0.0147s)


❯ be rails db:rollback:main
== 20220511151646 AddExcludeFromFreeUserCapToNamespaceSettings: reverting =====
-- transaction_open?()
   -> 0.0000s
-- remove_column(:namespace_settings, :exclude_from_free_user_cap)
   -> 0.0021s
== 20220511151646 AddExcludeFromFreeUserCapToNamespaceSettings: reverted (0.0146s)

MR acceptance checklist

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