Properly migrate automatic merge GitLab comments (#27873)

GitLab generates "system notes" whenever an event happens within the
platform. Unlike Gitea, those events are stored and retrieved as text
comments with no semantic details. The only way to tell whether a
comment was generated in this manner is the `system` flag on the note
type.

This PR adds detection for two specific kinds of events: Scheduling and
un-scheduling of automatic merges on a PR. When detected, they are
downloaded using Gitea's type for these events, and eventually uploaded
into Gitea in the expected format, i.e. with no text content in the
comment.

This PR also updates the template used to render comments to add support
for migrated comments of these two types.

ref:
https://gitlab.com/gitlab-org/gitlab/-/blob/11bd6dc826e0bea2832324a1d7356949a9398884/app/services/system_notes/merge_requests_service.rb#L6-L17

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/migrations/gitea_uploader.go

@@ -487,6 +487,8 @@ func (g *GiteaLocalUploader) CreateComments(comments ...*base.Comment) error {
 			if comment.Meta["NewTitle"] != nil {
 				cm.NewTitle = fmt.Sprintf("%s", comment.Meta["NewTitle"])
 			}
+		case issues_model.CommentTypePRScheduledToAutoMerge, issues_model.CommentTypePRUnScheduledToAutoMerge:
+			cm.Content = ""
 		default:
 		}