Cache emails and periodically try to re-send them if the operation fails

[alecello]

This may be a bit of a big one, but missed emails are kinda bad.

It would be nice if the server were able to cache the contents of the emails it sends (at least the ones sent to applicants) somewhere on the local filesystem (or at least in memory) and try to re-send them periodically if the SMTP server could not be contacted for some reason. This could work by implementing a queue in the database and implementing mail sending with a worker of some kind that consumes said queue. Since the email is a template anyways, the cache can be just the few parameters that are needed to fill it in.