Pivot Devops adoption to groups

What does this MR do?

Introduces new association to DevOps adoption segment.

We are in the process of pivoting DevOps adoption from multiple groups per segment to one. This first MR adds new structure which will be filled and used in upcoming MRs.

Please note that this MR is targeting devops-adoption-pivot branch, not master.

Migration output

== 20210121100038 AddDevopsAdoptionGroupSegment: reverting ====================
-- remove_index(:analytics_devops_adoption_segments, {:column=>:namespace_id})
   -> 0.0103s
-- remove_foreign_key(:analytics_devops_adoption_segments, :namespaces)
   -> 0.0081s
-- remove_column(:analytics_devops_adoption_segments, :namespace_id, :integer)
   -> 0.0015s
== 20210121100038 AddDevopsAdoptionGroupSegment: reverted (0.0381s) ===========

== 20210121100038 AddDevopsAdoptionGroupSegment: migrating ====================
-- add_column(:analytics_devops_adoption_segments, :namespace_id, :integer)
   -> 0.0030s
-- add_foreign_key(:analytics_devops_adoption_segments, :namespaces)
   -> 0.0057s
-- add_index(:analytics_devops_adoption_segments, :namespace_id, {:unique=>true})
Pivot Devops adoption to groups
   -> 0.0263s
== 20210121100038 AddDevopsAdoptionGroupSegment: migrated (0.0516s) ===========

Screenshots (strongly suggested)

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

Related to #297583 (closed)