After the change
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79227
the merge request creation form loses context of
the issue it was created from when a custom branch name
is used

This change adds an ability to retain the issue
IID context throughout the process so the text
'Closes #IssueIID' appears on the merge request
regardless of how the branch was created
(automatic name or custom name)

This brings back the feature of related merge requests
and system notes on the issue that went absent
after the change above

Changelog: fixed