Add namespace settings table

What does this MR do?

We need to store more and more data in namespace table - especially regarding different settings. To make things easier, Project settings table was introduced.

In order to add forking restriction in #216987 (closed) we need to add yet another setting to namespace. I thought maybe it's a good idea to introduce Namespace settings table to avoid growing namespace table into even bigger table.

Screenshots

== 20200703124823 CreateNamespaceSettings: migrating ==========================
-- create_table(:namespace_settings, {:id=>false})
   -> 0.0096s
== 20200703124823 CreateNamespaceSettings: migrated (0.0097s) =================

== 20200703124823 CreateNamespaceSettings: reverting ==========================
-- drop_table(:namespace_settings, {:id=>false})
   -> 0.0141s
== 20200703124823 CreateNamespaceSettings: reverted (0.0165s) =================

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