Allow directors to send apply reminder emails

[waalbert]

Add UI that will allow a director to send out "reminder to apply" emails to users that have signed in but haven't submitted an app (i.e. their roles field doesn't exist).

[taesungh]

Reference note: #179/#204 added consistent records for both login systems, so querying as {"last_login": {"$exists": True}, roles: {"$exists": False}} should be sufficient. This should also exclude bots (noted in #496) since they generally won't complete the full authentication flow.

In previous years, we just did a mass BCC email, but agree that having a UI would be nice. We might also need to store another property for email history if there's a plan to run this process multiple times to avoid resending to people who already got the notice.

[waalbert]

Closing as completed in #547