Move some more atomic values under Channels (#18837)

* Move some more atomic values under Channels

The following fields were moved:

```
asymmetricSigningKey atomic.Value
clientConfig         atomic.Value
clientConfigHash     atomic.Value
limitedClientConfig  atomic.Value
```

And also moved the initialization order from NewServer
to Channels.Start to better reflect the order of things.

Removed AsymmetricSigningKey from ConfigService
as it was no longer used.

Removed calling regenerateClientConfig during startup explicitly
because it was anyways called from ensureAsymmetricSigningKey.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=87df1e15-588e-49ff-8bd1-ffa9651b8c82
```release-note
NONE
```

* Fix lint error

```release-note
NONE
```

app/channels.go

@@ -4,8 +4,11 @@
 package app
 
 import (
+	"sync/atomic"
+
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/services/imageproxy"
+	"github.com/pkg/errors"
 )
 
 // Channels contains all channels related state.
@@ -14,6 +17,11 @@ type Channels struct {
 
 	imageProxy *imageproxy.ImageProxy
 
+	asymmetricSigningKey atomic.Value
+	clientConfig         atomic.Value
+	clientConfigHash     atomic.Value
+	limitedClientConfig  atomic.Value
+
 	// cached counts that are used during notice condition validation
 	cachedPostCount   int64
 	cachedUserCount   int64
@@ -35,10 +43,13 @@ func NewChannels(s *Server) (*Channels, error) {
 	}, nil
 }
 
-func (c *Channels) Start() error {
+func (ch *Channels) Start() error {
+	if err := ch.ensureAsymmetricSigningKey(); err != nil {
+		return errors.Wrapf(err, "unable to ensure asymmetric signing key")
+	}
 	return nil
 }
 
-func (c *Channels) Stop() error {
+func (*Channels) Stop() error {
 	return nil
 }