Split "Pipelines for merged results" and "Merge Train" check boxes

What does this MR do?

This MR splits 2 checkboxes that were once combined.

The checkboxes enable pipelines for merged results and merge trains respectively.

'Merge trains' requires 'pipelines for merged results' to be enabled to work. In further iterations, we will disable the merge trains checkbox when pipelines for merged results is disabled.

Migrations output

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❯ VERSION=20201112195322 be rake db:migrate:up
== 20201112195322 ReseedMergeTrainsEnabled: migrating =========================
-- transaction_open?()
   -> 0.0000s
-- exec_query("SELECT COUNT(*) AS count FROM \"project_ci_cd_settings\" WHERE \"project_ci_cd_settings\".\"merge_pipelines_enabled\" = TRUE")
   -> 0.0063s
== 20201112195322 ReseedMergeTrainsEnabled: migrated (0.0065s) ================

❯ VERSION=20201112195322 be rake db:migrate:down
== 20201112195322 ReseedMergeTrainsEnabled: reverting =========================
== 20201112195322 ReseedMergeTrainsEnabled: reverted (0.0000s) ================

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 #244831 (closed)