Add a group setting to allow Maintainers to create sub-groups

What does this MR do?

Currently only owners can create sub-groups. This MR makes it so that groups can be configured to allow maintainers to also have the rights to create sub-groups. This option should be enabled by default for new groups, but disabled for existing groups. Addresses issue #51610 (closed).

Output of migrations

== 20190626175626 AddGroupCreationLevelToNamespaces: migrating ================
-- add_column(:namespaces, :subgroup_creation_level, :integer)
   -> 0.0672s
-- change_column_default(:namespaces, :subgroup_creation_level, 1)
   -> 0.0628s
== 20190626175626 AddGroupCreationLevelToNamespaces: migrated (0.1303s) =======

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Database checklist

• Conforms to the database guides

Adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body

database