Do some performance optimize for issues list and view issue/pull (gitea#29515)

This PR do some performance optimzations.

- [x] Add `index` for the column `comment_id` of `Attachment` table to
accelerate query from the database.
- [x] Remove unnecessary database queries when viewing issues. Before
some conditions which id = 0 will be sent to the database
- [x] Remove duplicated load posters
- [x] Batch loading attachements, isread of comments on viewing issue

---------

Co-authored-by: Zettat123 <zettat123@gmail.com>
Conflicts:
models/issues/comment_code.go: function was renamed in Forgejo
models/migrations/migrations.go: migration already ported

models/issues/issue_list.go

@@ -391,9 +391,8 @@ func (issues IssueList) LoadAttachments(ctx context.Context) (err error) {
 		if left < limit {
 			limit = left
 		}
-		rows, err := db.GetEngine(ctx).Table("attachment").
-			Join("INNER", "issue", "issue.id = attachment.issue_id").
-			In("issue.id", issuesIDs[:limit]).
+		rows, err := db.GetEngine(ctx).
+			In("issue_id", issuesIDs[:limit]).
 			Rows(new(repo_model.Attachment))
 		if err != nil {
 			return err
@@ -612,3 +611,23 @@ func (issues IssueList) GetApprovalCounts(ctx context.Context) (map[int64][]*Rev
 
 	return approvalCountMap, nil
 }
+
+func (issues IssueList) LoadIsRead(ctx context.Context, userID int64) error {
+	issueIDs := issues.getIssueIDs()
+	issueUsers := make([]*IssueUser, 0, len(issueIDs))
+	if err := db.GetEngine(ctx).Where("uid =?", userID).
+		In("issue_id").
+		Find(&issueUsers); err != nil {
+		return err
+	}
+
+	for _, issueUser := range issueUsers {
+		for _, issue := range issues {
+			if issue.ID == issueUser.IssueID {
+				issue.IsRead = issueUser.IsRead
+			}
+		}
+	}
+
+	return nil
+}