Add group crm settings

What does this MR do and why?

Related to #2256

We need somewhere to store whether Customer relations (CRM) is enabled for a group.

I have a feeling adding columns to namespaces might be challenging because of the size?

We also will probably want to add other settings moving forward- for example:

• instead of just off/on, we might want to set access level (e.g. report/developer+ etc)
• we may want to control who can "admin" crm records (e.g. create contacts/organizations)

Also, as it is likely to be a small number/%age of namespaces enabling this option, it seemed sensible to separate into it's own table

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:up VERSION=20211216220939 RAILS_ENV=development
== 20211216220939 AddGroupCrmSettings: migrating ==========================
-- create_table(:group_crm_settings, {:id=>false})
   -> 0.0136s
== 20211216220939 AddGroupCrmSettings: migrated (0.0137s) =================

lee@cc-gdk-2:~/gitlab-development-kit/gitlab$ bundle exec rake db:migrate:down VERSION=20211216220939 RAILS_ENV=development
== 20211216220939 AddGroupCrmSettings: reverting ==========================
-- drop_table(:group_crm_settings, {:id=>false})
   -> 0.0033s
== 20211216220939 AddGroupCrmSettings: reverted (0.0045s) =================

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