Improve queue & process & stacktrace (#24636)

Although some features are mixed together in this PR, this PR is not
that large, and these features are all related.

Actually there are more than 70 lines are for a toy "test queue", so
this PR is quite simple.

Major features:

1. Allow site admin to clear a queue (remove all items in a queue)
* Because there is no transaction, the "unique queue" could be corrupted
in rare cases, that's unfixable.
* eg: the item is in the "set" but not in the "list", so the item would
never be able to be pushed into the queue.
* Now site admin could simply clear the queue, then everything becomes
correct, the lost items could be re-pushed into queue by future
operations.
3. Split the "admin/monitor" to separate pages
4. Allow to download diagnosis report
* In history, there were many users reporting that Gitea queue gets
stuck, or Gitea's CPU is 100%
    * With diagnosis report, maintainers could know what happens clearly

The diagnosis report sample:
[gitea-diagnosis-20230510-192913.zip](11441346/gitea-diagnosis-20230510-192913.zip)
, use "go tool pprof profile.dat" to view the report.


Screenshots:


![image](320659b4-2eda-4def-8dc0-5ea08d578063)


![image](c5c46fae-9dc0-44ca-8cd3-57beedc5035e)


![image](6168a811-42a1-4e64-a263-0617a6c8c4fe)

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/queue/manager.go

@@ -33,6 +33,9 @@ type ManagedWorkerPoolQueue interface {
 	// FlushWithContext tries to make the handler process all items in the queue synchronously.
 	// It is for testing purpose only. It's not designed to be used in a cluster.
 	FlushWithContext(ctx context.Context, timeout time.Duration) error
+
+	// RemoveAllItems removes all items in the base queue (on-the-fly items are not affected)
+	RemoveAllItems(ctx context.Context) error
 }
 
 var manager *Manager