Add Ddr::Alerts functionality

Re: configuration -- it seems like you could have the app (dul-hydra or ddr-public) config the "message context" for ddr-alerts to the relevant value at https://github.com/duke-libraries/ddr-alerts/blob/develop/lib/ddr/alerts/message_context.rb#L5-L6.  Then the partial (or helper) just uses the config value to scope the messages.