Move pluginCommands into Channels (#18974)

* Move pluginCommands into Channels

We move pluginCommands, pluginCommandsLock
into Channels.

We also move the plugin related route handlers
under Channels and move the init code under
NewChannels. To achieve this, the router initialization
is bumped up.

Along with it, we clean up some App methods
which were just wrappers over Channel methods.
Instead, we call the Channel method directly
to make things more readable and easy to understand.

```release-note
NONE
```

* fix tests

```release-note
NONE
```

app/plugin_install.go

@@ -61,10 +61,6 @@ const managedPluginFileName = ".filestore"
 // fileStorePluginFolder is the folder name in the file store of the plugin bundles installed.
 const fileStorePluginFolder = "plugins"
 
-func (a *App) InstallPluginFromData(data model.PluginEventData) {
-	a.ch.installPluginFromData(data)
-}
-
 func (ch *Channels) installPluginFromData(data model.PluginEventData) {
 	mlog.Debug("Installing plugin as per cluster message", mlog.String("plugin_id", data.Id))
 
@@ -111,10 +107,6 @@ func (ch *Channels) installPluginFromData(data model.PluginEventData) {
 	}
 }
 
-func (a *App) RemovePluginFromData(data model.PluginEventData) {
-	a.ch.removePluginFromData(data)
-}
-
 func (ch *Channels) removePluginFromData(data model.PluginEventData) {
 	mlog.Debug("Removing plugin as per cluster message", mlog.String("plugin_id", data.Id))
 
@@ -128,10 +120,6 @@ func (ch *Channels) removePluginFromData(data model.PluginEventData) {
 }
 
 // InstallPluginWithSignature verifies and installs plugin.
-func (a *App) InstallPluginWithSignature(pluginFile, signature io.ReadSeeker) (*model.Manifest, *model.AppError) {
-	return a.ch.installPluginWithSignature(pluginFile, signature)
-}
-
 func (ch *Channels) installPluginWithSignature(pluginFile, signature io.ReadSeeker) (*model.Manifest, *model.AppError) {
 	return ch.installPlugin(pluginFile, signature, installPluginLocallyAlways)
 }
@@ -189,11 +177,7 @@ func (ch *Channels) installPlugin(pluginFile, signature io.ReadSeeker, installat
 
 // InstallMarketplacePlugin installs a plugin listed in the marketplace server. It will get the plugin bundle
 // from the prepackaged folder, if available, or remotely if EnableRemoteMarketplace is true.
-func (a *App) InstallMarketplacePlugin(request *model.InstallMarketplacePluginRequest) (*model.Manifest, *model.AppError) {
-	return a.ch.installMarketplacePlugin(request)
-}
-
-func (ch *Channels) installMarketplacePlugin(request *model.InstallMarketplacePluginRequest) (*model.Manifest, *model.AppError) {
+func (ch *Channels) InstallMarketplacePlugin(request *model.InstallMarketplacePluginRequest) (*model.Manifest, *model.AppError) {
 	var pluginFile, signatureFile io.ReadSeeker
 
 	prepackagedPlugin, appErr := ch.getPrepackagedPlugin(request.Id, request.Version)
@@ -257,10 +241,6 @@ const (
 	installPluginLocallyAlways
 )
 
-func (a *App) installPluginLocally(pluginFile, signature io.ReadSeeker, installationStrategy pluginInstallationStrategy) (*model.Manifest, *model.AppError) {
-	return a.ch.installPluginLocally(pluginFile, signature, installationStrategy)
-}
-
 func (ch *Channels) installPluginLocally(pluginFile, signature io.ReadSeeker, installationStrategy pluginInstallationStrategy) (*model.Manifest, *model.AppError) {
 	pluginsEnvironment := ch.GetPluginsEnvironment()
 	if pluginsEnvironment == nil {
@@ -412,11 +392,7 @@ func (ch *Channels) installExtractedPlugin(manifest *model.Manifest, fromPluginD
 	return manifest, nil
 }
 
-func (a *App) RemovePlugin(id string) *model.AppError {
-	return a.ch.removePlugin(id)
-}
-
-func (ch *Channels) removePlugin(id string) *model.AppError {
+func (ch *Channels) RemovePlugin(id string) *model.AppError {
 	// Disable plugin before removal to make sure this
 	// plugin remains disabled on re-install.
 	if err := ch.disablePlugin(id); err != nil {
@@ -457,10 +433,6 @@ func (ch *Channels) removePlugin(id string) *model.AppError {
 	return nil
 }
 
-func (a *App) removePluginLocally(id string) *model.AppError {
-	return a.ch.removePluginLocally(id)
-}
-
 func (ch *Channels) removePluginLocally(id string) *model.AppError {
 	pluginsEnvironment := ch.GetPluginsEnvironment()
 	if pluginsEnvironment == nil {
@@ -488,7 +460,7 @@ func (ch *Channels) removePluginLocally(id string) *model.AppError {
 
 	pluginsEnvironment.Deactivate(id)
 	pluginsEnvironment.RemovePlugin(id)
-	ch.srv.unregisterPluginCommands(id)
+	ch.unregisterPluginCommands(id)
 
 	if err := os.RemoveAll(pluginPath); err != nil {
 		return model.NewAppError("removePlugin", "app.plugin.remove.app_error", nil, err.Error(), http.StatusInternalServerError)