Allow mentioning user in the channel welcome messages

[jfrerich]

When configuring Team welcome messages, the config allows referencing fields from the MessageTemplate struct. However, setting a welcome message through /welcomebot set_channel_welcome only allows text. This ticket is asking to allow accessing the members of the MessageTemplate struct while creating a channel welcomebot message.

type MessageTemplate struct {
    WelcomeBot      *model.User
    User            *model.User
    Team            *model.Team
    Townsquare      *model.Channel
    DirectMessage   *model.Channel
    UserDisplayName string
}

The implementation is up for discussion, but one idea is to somehow incorporate inline templating similar to the usage found in the config.

Example: "### Welcome {{.UserDisplayName}} to the Staff {{.Team.DisplayName}} team!",