Create async mailing capability (17)

When a lot of e-mails are being sent, this can take a very long time, which 'blocks' the application until sending the e-mails is done. The solution will be to handle the e-mails asynchronously in a queue. The e-mail queue will be a fourth top-level concern.

Having a top-level concern for e-mails might make implementing #7 (closed) more easy as it can all be done in one place.

• Create async mail service (5)
• Create STOMP channel for receiving (subscribing to) async mail service status (3)
• Make front-end progress interface for sending e-mails (2)
• Make front-end modal showing extended progress information (3)
• Change existing endpoints that send e-mails to non-blocking endpoints emitting to async mail service (removing existing exception handlers) (4)

Additional tasks added later:

• Think of a better way to present mail functionality to the user when there is no active configuration.
• Fix ESLint errors caused by the new implementation.
• Remove any legacy code/configuration of the old mail system, e.g., remove spring.mail from application.yml.
• Fix checkstyle errors caused by the new implementation.