MM-58038: Use context to call master for DeletePost (#27098)

Calling app.DeletePost immediately after creating a post
is susceptible to replica lag because we were calling the
replica to check for the post.

We fix this by passing a context to always query master.

https://mattermost.atlassian.net/browse/MM-58038

```release-note
NONE
```

server/channels/app/post_metadata.go

@@ -645,7 +645,7 @@ func (a *App) getLinkMetadata(c request.CTX, requestURL string, timestamp int64,
 	if looksLikeAPermalink(requestURL, a.GetSiteURL()) && *a.Config().ServiceSettings.EnablePermalinkPreviews {
 		referencedPostID := requestURL[len(requestURL)-26:]
 
-		referencedPost, appErr := a.GetSinglePost(referencedPostID, false)
+		referencedPost, appErr := a.GetSinglePost(c, referencedPostID, false)
 		// TODO: Look into saving a value in the LinkMetadata.Data field to prevent perpetually re-querying for the deleted post.
 		if appErr != nil {
 			return nil, nil, nil, appErr