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

routers/api/v1/repo/issue_comment.go

@@ -323,10 +323,6 @@ func ListRepoIssueComments(ctx *context.APIContext) {
 		ctx.Error(http.StatusInternalServerError, "LoadIssues", err)
 		return
 	}
-	if err := comments.LoadPosters(ctx); err != nil {
-		ctx.Error(http.StatusInternalServerError, "LoadPosters", err)
-		return
-	}
 	if err := comments.LoadAttachments(ctx); err != nil {
 		ctx.Error(http.StatusInternalServerError, "LoadAttachments", err)
 		return