[MM-7968] restrict creation of direct channels to team members (#17222)

* restrict creation of direct channels to team members

* run make i18n-extract

* add suggestions from hahmadia

* place common-team-check logic in app layer

* use flat SQL query

* show more specific error message to user

* MM-7968: Fmt file.

* MM-7968: Fix for moved session field.

Co-authored-by: Max Erenberg <max.erenberg@mattermost.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Martin Kraft <martinkraft@gmail.com>
Co-authored-by: Martin Kraft <martin@upspin.org>

app/slashcommands/command_msg.go

@@ -85,7 +85,7 @@ func (*msgProvider) DoCommand(a *app.App, c *request.Context, args *model.Comman
 			var directChannel *model.Channel
 			if directChannel, err = a.GetOrCreateDirectChannel(c, args.UserId, userProfile.Id); err != nil {
 				mlog.Error(err.Error())
-				return &model.CommandResponse{Text: args.T("api.command_msg.dm_fail.app_error"), ResponseType: model.COMMAND_RESPONSE_TYPE_EPHEMERAL}
+				return &model.CommandResponse{Text: args.T(err.Id), ResponseType: model.COMMAND_RESPONSE_TYPE_EPHEMERAL}
 			}
 			targetChannelId = directChannel.Id
 		} else {