Merge When Pipeline Succeeds (MWPS) cannot be chosen while Merge Trains option is enabled

Problem

Generally, in merge requests, user can choose one merge strategy from 1. "Merge When Pipeline Succeeds" 2. "Merge Immediately".

And with https://gitlab.com/gitlab-org/gitlab-ee/issues/9186, when the merge train option is enabled, user can choose one merge strategy from 1. "Add Merge Train" (or "Add To Merge Train When Pipeline Succeeds") 2. "Merge Immediately". This effectively hides the "Merge When Pipeline Succeeds" option, however, users might still want to use this functionality as well as they need "Merge Immediately" option.

Proposal

When Merge train option is enabled, show three merge strategies:

1. "Add Merge Train" (or "Add To Merge Train When Pipeline Succeeds")
2. "Merge When Pipeline Succeeds"
3. "Merge Immediately"

When Merge train option is disabled or license is not sufficient, show two merge strategies:

1. "Merge When Pipeline Succeeds"
2. "Merge Immediately"