Do some performance optimize for issues list and view issue/pull (#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>

routers/web/repo/pull_review.go

@@ -179,11 +179,9 @@ func renderConversation(ctx *context.Context, comment *issues_model.Comment, ori
 		return
 	}
 
-	for _, c := range comments {
-		if err := c.LoadAttachments(ctx); err != nil {
-			ctx.ServerError("LoadAttachments", err)
-			return
-		}
+	if err := comments.LoadAttachments(ctx); err != nil {
+		ctx.ServerError("LoadAttachments", err)
+		return
 	}
 
 	ctx.Data["IsAttachmentEnabled"] = setting.Attachment.Enabled