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_test.go

@@ -106,7 +106,7 @@ func TestPluginCommand(t *testing.T) {
 			require.NotEqual(t, "plugin", commands.Trigger)
 		}
 
-		th.App.RemovePlugin(pluginIDs[0])
+		th.App.ch.RemovePlugin(pluginIDs[0])
 	})
 
 	t.Run("re-entrant command registration on config change", func(t *testing.T) {
@@ -207,7 +207,7 @@ func TestPluginCommand(t *testing.T) {
 			killed = true
 		}
 
-		th.App.RemovePlugin(pluginIDs[0])
+		th.App.ch.RemovePlugin(pluginIDs[0])
 		require.False(t, killed, "execute command appears to have deadlocked")
 	})
 
@@ -285,7 +285,7 @@ func TestPluginCommand(t *testing.T) {
 		require.Equal(t, model.CommandResponseTypeEphemeral, resp.ResponseType)
 		require.Equal(t, "text", resp.Text)
 
-		th.App.RemovePlugin(pluginIDs[0])
+		th.App.ch.RemovePlugin(pluginIDs[0])
 	})
 	t.Run("plugin has crashed before execution of command", func(t *testing.T) {
 		tearDown, pluginIDs, activationErrors := SetAppEnvironmentWithPlugins(t, []string{`
@@ -329,7 +329,7 @@ func TestPluginCommand(t *testing.T) {
 		require.Nil(t, resp)
 		require.NotNil(t, err)
 		require.Equal(t, err.Id, "model.plugin_command_error.error.app_error")
-		th.App.RemovePlugin(pluginIDs[0])
+		th.App.ch.RemovePlugin(pluginIDs[0])
 	})
 
 	t.Run("plugin has crashed due to the execution of the command", func(t *testing.T) {
@@ -374,7 +374,7 @@ func TestPluginCommand(t *testing.T) {
 		require.Nil(t, resp)
 		require.NotNil(t, err)
 		require.Equal(t, err.Id, "model.plugin_command_crash.error.app_error")
-		th.App.RemovePlugin(pluginIDs[0])
+		th.App.ch.RemovePlugin(pluginIDs[0])
 	})
 
 }