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_commands.go

@@ -54,10 +54,10 @@ func (a *App) RegisterPluginCommand(pluginID string, command *model.Command) err
 		AutocompleteIconData: command.AutocompleteIconData,
 	}
 
-	a.Srv().pluginCommandsLock.Lock()
-	defer a.Srv().pluginCommandsLock.Unlock()
+	a.ch.pluginCommandsLock.Lock()
+	defer a.ch.pluginCommandsLock.Unlock()
 
-	for _, pc := range a.Srv().pluginCommands {
+	for _, pc := range a.ch.pluginCommands {
 		if pc.Command.Trigger == command.Trigger && pc.Command.TeamId == command.TeamId {
 			if pc.PluginId == pluginID {
 				pc.Command = command
@@ -66,7 +66,7 @@ func (a *App) RegisterPluginCommand(pluginID string, command *model.Command) err
 		}
 	}
 
-	a.Srv().pluginCommands = append(a.Srv().pluginCommands, &PluginCommand{
+	a.ch.pluginCommands = append(a.ch.pluginCommands, &PluginCommand{
 		Command:  command,
 		PluginId: pluginID,
 	})
@@ -76,41 +76,37 @@ func (a *App) RegisterPluginCommand(pluginID string, command *model.Command) err
 func (a *App) UnregisterPluginCommand(pluginID, teamID, trigger string) {
 	trigger = strings.ToLower(trigger)
 
-	a.Srv().pluginCommandsLock.Lock()
-	defer a.Srv().pluginCommandsLock.Unlock()
+	a.ch.pluginCommandsLock.Lock()
+	defer a.ch.pluginCommandsLock.Unlock()
 
 	var remaining []*PluginCommand
-	for _, pc := range a.Srv().pluginCommands {
+	for _, pc := range a.ch.pluginCommands {
 		if pc.Command.TeamId != teamID || pc.Command.Trigger != trigger {
 			remaining = append(remaining, pc)
 		}
 	}
-	a.Srv().pluginCommands = remaining
+	a.ch.pluginCommands = remaining
 }
 
-func (a *App) UnregisterPluginCommands(pluginID string) {
-	a.Srv().unregisterPluginCommands(pluginID)
-}
-
-func (s *Server) unregisterPluginCommands(pluginID string) {
-	s.pluginCommandsLock.Lock()
-	defer s.pluginCommandsLock.Unlock()
+func (ch *Channels) unregisterPluginCommands(pluginID string) {
+	ch.pluginCommandsLock.Lock()
+	defer ch.pluginCommandsLock.Unlock()
 
 	var remaining []*PluginCommand
-	for _, pc := range s.pluginCommands {
+	for _, pc := range ch.pluginCommands {
 		if pc.PluginId != pluginID {
 			remaining = append(remaining, pc)
 		}
 	}
-	s.pluginCommands = remaining
+	ch.pluginCommands = remaining
 }
 
 func (a *App) PluginCommandsForTeam(teamID string) []*model.Command {
-	a.Srv().pluginCommandsLock.RLock()
-	defer a.Srv().pluginCommandsLock.RUnlock()
+	a.ch.pluginCommandsLock.RLock()
+	defer a.ch.pluginCommandsLock.RUnlock()
 
 	var commands []*model.Command
-	for _, pc := range a.Srv().pluginCommands {
+	for _, pc := range a.ch.pluginCommands {
 		if pc.Command.TeamId == "" || pc.Command.TeamId == teamID {
 			commands = append(commands, pc.Command)
 		}
@@ -126,14 +122,14 @@ func (a *App) tryExecutePluginCommand(c *request.Context, args *model.CommandArg
 	trigger = strings.ToLower(trigger)
 
 	var matched *PluginCommand
-	a.Srv().pluginCommandsLock.RLock()
-	for _, pc := range a.Srv().pluginCommands {
+	a.ch.pluginCommandsLock.RLock()
+	for _, pc := range a.ch.pluginCommands {
 		if (pc.Command.TeamId == "" || pc.Command.TeamId == args.TeamId) && pc.Command.Trigger == trigger {
 			matched = pc
 			break
 		}
 	}
-	a.Srv().pluginCommandsLock.RUnlock()
+	a.ch.pluginCommandsLock.RUnlock()
 	if matched == nil {
 		return nil, nil, nil
 	}