[MM-23727] Make channel validation consistent on the server (#14230)

* MM-23727 Ensure user ids not allowed in channel name:

MM-23727 Move channel name validation to model level

* MM-23727 Update wording

app/channel.go

@@ -144,10 +144,6 @@ func (a *App) CreateChannelWithUser(channel *model.Channel, userId string) (*mod
 		return nil, model.NewAppError("CreateChannelWithUser", "api.channel.create_channel.direct_channel.app_error", nil, "", http.StatusBadRequest)
 	}
 
-	if strings.Index(channel.Name, "__") > 0 {
-		return nil, model.NewAppError("CreateChannelWithUser", "api.channel.create_channel.invalid_character.app_error", nil, "", http.StatusBadRequest)
-	}
-
 	if len(channel.TeamId) == 0 {
 		return nil, model.NewAppError("CreateChannelWithUser", "app.channel.create_channel.no_team_id.app_error", nil, "", http.StatusBadRequest)
 	}
@@ -464,14 +460,6 @@ func (a *App) GetGroupChannel(userIds []string) (*model.Channel, *model.AppError
 
 // UpdateChannel updates a given channel by its Id. It also publishes the CHANNEL_UPDATED event.
 func (a *App) UpdateChannel(channel *model.Channel) (*model.Channel, *model.AppError) {
-	userIds := strings.Split(channel.Name, "__")
-	if channel.Type != model.CHANNEL_DIRECT &&
-		len(userIds) == 2 &&
-		model.IsValidId(userIds[0]) &&
-		model.IsValidId(userIds[1]) {
-		return nil, model.NewAppError("UpdateChannel", "api.channel.update_channel.invalid_character.app_error", nil, "", http.StatusBadRequest)
-	}
-
 	_, err := a.Srv().Store.Channel().Update(channel)
 	if err != nil {
 		return nil, err