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

Problem to solve

The existence of a single checkbox that enables both "Pipelines for merged results" and "Merge Train" does not allow users to selectively enable only "Pipelines for merged results" or only "Merge Train".

Further details

In GitLab 12.0, Merge Trains was shipped. At that time, Pipelines for merged results and Merge Train check boxes were separate and users were able to disable "Merge Train" even if "Pipelines for merged results" was enabled. Merge Train was disabled by default because it had a scalability issue.

In GitLab 12.1, Merge Trains was enabled whenever "Pipelines for merged results" is enabled because the scalability issue was resolved. "Pipelines for merged results" and "Merge Train" check boxes were combined into a single check box.

One reason for the problem to solve is stated in gitlab-com/gl-infra/delivery#1107 (closed) that users may not want to enable "Merge Trains" due to the CI cost increase.

Proposal

• Separate the "Pipelines for merged results" and "Merge Train" into 2 checkboxes. For the reference, you can see the design at GitLab 12.0 below.