update: add warning task to send notices to users with expiring ECA documents

• update: add warning task to send notices to users with expiring ECA documents

Adds a task that handles short and long period notices for document expiration. This also adds a test that should cover all of the standard use cases for the task.

This also includes a placeholder email template until the final template is approved. It will be replaced in a later pull request.

Resolves #6