Resolve "Prevent forking outside a group"

What does this MR do?

In this MR my goal was to create new setting in groups to mark if any of child project on any level can be forked outside of the group.

This was previously set only for groups with group managed accounts - I need to preserve this setting for now and in background migration migrate it to the group level.

In future iteration we should get rid of this setting in saml_provider table.

Screenshots

DB migration:

== 20200717080257 AddPreventForkingToNamespaceSettings: migrating =============
-- add_column(:namespace_settings, :prevent_forking_outside_group, :boolean, {:null=>false, :default=>false})
   -> 0.0075s
== 20200717080257 AddPreventForkingToNamespaceSettings: migrated (0.0077s) ====

== 20200717080257 AddPreventForkingToNamespaceSettings: reverting =============
-- remove_column(:namespace_settings, :prevent_forking_outside_group, :boolean, {:null=>false, :default=>false})
   -> 0.0184s
== 20200717080257 AddPreventForkingToNamespaceSettings: reverted (0.0228s) ====

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #216987 (closed)