Resolve "[DevOps Adoption] Migrate existing segments to groups"

What does this MR do?

Adds data migration which moves DevopsAdoption existing data from segments with multiple groups to segments with single group only.

Migration output

``` == 20210203143131 MigrateExistingDevopsSegmentsToGroups: reverting ============ This data migration is irreversible == 20210203143131 MigrateExistingDevopsSegmentsToGroups: reverted (0.0000s) ===

== 20210203143131 MigrateExistingDevopsSegmentsToGroups: migrating ============ == 20210203143131 MigrateExistingDevopsSegmentsToGroups: migrated (0.0193s) ===

</details>

## Screenshots  (strongly suggested)

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## Does this MR meet the acceptance criteria?

### Conformity

- [ ] [Changelog entry](https://docs.gitlab.com/ee/development/changelog.html) 
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- [ ] [Style guides](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/doc/development/contributing/style_guides.md)
- [ ] [Database guides](https://docs.gitlab.com/ee/development/database_review.html)
- [ ] [Separation of EE specific content](https://docs.gitlab.com/ee/development/ee_features.html#separation-of-ee-code)

### Availability and Testing

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- [ ] [Review and add/update tests for this feature/bug](https://docs.gitlab.com/ee/development/testing_guide/index.html). Consider [all test levels](https://docs.gitlab.com/ee/development/testing_guide/testing_levels.html). See the [Test Planning Process](https://about.gitlab.com/handbook/engineering/quality/test-engineering).
- [ ] [Tested in all supported browsers](https://docs.gitlab.com/ee/install/requirements.html#supported-web-browsers)
- [ ] Informed Infrastructure department of a default or new setting change, if applicable per [definition of done](https://docs.gitlab.com/ee/development/contributing/merge_request_workflow.html#definition-of-done)

### Security

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- [ ] 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