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/repo/attachment.go

@@ -24,7 +24,7 @@ type Attachment struct {
 	IssueID           int64  `xorm:"INDEX"`           // maybe zero when creating
 	ReleaseID         int64  `xorm:"INDEX"`           // maybe zero when creating
 	UploaderID        int64  `xorm:"INDEX DEFAULT 0"` // Notice: will be zero before this column added
-	CommentID         int64
+	CommentID         int64  `xorm:"INDEX"`
 	Name              string
 	DownloadCount     int64              `xorm:"DEFAULT 0"`
 	Size              int64              `xorm:"DEFAULT 0"`