Moving slash-commands out of app package (#14979)

* Moving slash-commands out of app package

* Fixing golint checks

* Fixing golangci-lint

* Fixing golangci-lint errors

app/slashcommands/command_settings.go

@@ -0,0 +1,43 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package slashcommands
+
+import (
+	goi18n "github.com/mattermost/go-i18n/i18n"
+	"github.com/mattermost/mattermost-server/v5/app"
+	"github.com/mattermost/mattermost-server/v5/model"
+)
+
+type SettingsProvider struct {
+}
+
+const (
+	CMD_SETTINGS = "settings"
+)
+
+func init() {
+	app.RegisterCommandProvider(&SettingsProvider{})
+}
+
+func (settings *SettingsProvider) GetTrigger() string {
+	return CMD_SETTINGS
+}
+
+func (settings *SettingsProvider) GetCommand(a *app.App, T goi18n.TranslateFunc) *model.Command {
+	return &model.Command{
+		Trigger:          CMD_SETTINGS,
+		AutoComplete:     true,
+		AutoCompleteDesc: T("api.command_settings.desc"),
+		AutoCompleteHint: "",
+		DisplayName:      T("api.command_settings.name"),
+	}
+}
+
+func (settings *SettingsProvider) DoCommand(a *app.App, args *model.CommandArgs, message string) *model.CommandResponse {
+	// This command is handled client-side and shouldn't hit the server.
+	return &model.CommandResponse{
+		Text:         args.T("api.command_settings.unsupported.app_error"),
+		ResponseType: model.COMMAND_RESPONSE_TYPE_EPHEMERAL,
+	}
+}