Remove the remaining fields from *app.Server (#19113)

We move these fields to Channels:
```
uploadLockMapMut sync.Mutex
uploadLockMap    map[string]bool
imgDecoder *imaging.Decoder
imgEncoder *imaging.Encoder
dndTaskMut sync.Mutex
dndTask    *model.ScheduledTask
```

I think this PR should conclue the initial phase
of migrating stuff from Server to Channels.

The remaining task would be to focus on continue
to create the remaining services from the
common things like users, teams, push notifications,
clustering for other products to consume.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4/87df1e15-588e-49ff-8bd1-ffa9651b8c82

```release-note
NONE
```

app/server.go

@@ -38,7 +38,6 @@ import (
 
 	"github.com/mattermost/mattermost-server/v6/app/email"
 	"github.com/mattermost/mattermost-server/v6/app/featureflag"
-	"github.com/mattermost/mattermost-server/v6/app/imaging"
 	"github.com/mattermost/mattermost-server/v6/app/request"
 	"github.com/mattermost/mattermost-server/v6/app/teams"
 	"github.com/mattermost/mattermost-server/v6/app/users"
@@ -174,23 +173,11 @@ type Server struct {
 
 	tracer *tracing.Tracer
 
-	// These are used to prevent concurrent upload requests
-	// for a given upload session which could cause inconsistencies
-	// and data corruption.
-	uploadLockMapMut sync.Mutex
-	uploadLockMap    map[string]bool
-
 	featureFlagSynchronizer      *featureflag.Synchronizer
 	featureFlagStop              chan struct{}
 	featureFlagStopped           chan struct{}
 	featureFlagSynchronizerMutex sync.Mutex
 
-	imgDecoder *imaging.Decoder
-	imgEncoder *imaging.Encoder
-
-	dndTaskMut sync.Mutex
-	dndTask    *model.ScheduledTask
-
 	products map[string]Product
 }
 
@@ -207,7 +194,6 @@ func NewServer(options ...Option) (*Server, error) {
 		},
 		licenseListeners: map[string]func(*model.License, *model.License){},
 		hashSeed:         maphash.MakeSeed(),
-		uploadLockMap:    map[string]bool{},
 		timezones:        timezones.New(),
 		products:         make(map[string]Product),
 	}
@@ -492,20 +478,6 @@ func NewServer(options ...Option) (*Server, error) {
 
 	s.setupFeatureFlags()
 
-	var imgErr error
-	s.imgDecoder, imgErr = imaging.NewDecoder(imaging.DecoderOptions{
-		ConcurrencyLevel: runtime.NumCPU(),
-	})
-	if imgErr != nil {
-		return nil, errors.Wrap(imgErr, "failed to create image decoder")
-	}
-	s.imgEncoder, imgErr = imaging.NewEncoder(imaging.EncoderOptions{
-		ConcurrencyLevel: runtime.NumCPU(),
-	})
-	if imgErr != nil {
-		return nil, errors.Wrap(imgErr, "failed to create image encoder")
-	}
-
 	s.initJobs()
 
 	s.clusterLeaderListenerId = s.AddClusterLeaderChangedListener(func() {
@@ -1027,12 +999,6 @@ func (s *Server) Shutdown() {
 		}
 	}
 
-	s.dndTaskMut.Lock()
-	if s.dndTask != nil {
-		s.dndTask.Cancel()
-	}
-	s.dndTaskMut.Unlock()
-
 	mlog.Info("Server stopped")
 
 	// Stop products.
@@ -2257,18 +2223,18 @@ func (s *Server) ReadFile(path string) ([]byte, *model.AppError) {
 }
 
 func createDNDStatusExpirationRecurringTask(a *App) {
-	a.ch.srv.dndTaskMut.Lock()
-	a.ch.srv.dndTask = model.CreateRecurringTaskFromNextIntervalTime("Unset DND Statuses", a.UpdateDNDStatusOfUsers, 5*time.Minute)
-	a.ch.srv.dndTaskMut.Unlock()
+	a.ch.dndTaskMut.Lock()
+	a.ch.dndTask = model.CreateRecurringTaskFromNextIntervalTime("Unset DND Statuses", a.UpdateDNDStatusOfUsers, 5*time.Minute)
+	a.ch.dndTaskMut.Unlock()
 }
 
 func cancelDNDStatusExpirationRecurringTask(a *App) {
-	a.ch.srv.dndTaskMut.Lock()
-	if a.ch.srv.dndTask != nil {
-		a.ch.srv.dndTask.Cancel()
-		a.ch.srv.dndTask = nil
+	a.ch.dndTaskMut.Lock()
+	if a.ch.dndTask != nil {
+		a.ch.dndTask.Cancel()
+		a.ch.dndTask = nil
 	}
-	a.ch.srv.dndTaskMut.Unlock()
+	a.ch.dndTaskMut.Unlock()
 }
 
 func runDNDStatusExpireJob(a *App) {