Alert journalist users about security issues

[eloquence]

Closely related to #22, we may also want to consider alerting journalist users about high severity security issues:

• so they can alert admins directly (in addition to any emails sent by the system);
• so they can potentially pause source communications until a security investigation has been completed.

This will require careful UX consideration to ensure we avoid alert fatigue, and to make alerts actionable.

[eloquence]

See also freedomofpress/securedrop-client#1182 for a similar idea to implement system alerts in the client app. Given that security alerts could potentially be triggered anywhere in the system, keeping this issue open for now to track at the system-level.

[ninavizz]

Shd we discuss in the near term @eloquence?

[zenmonkeykstop]

A prerequisite for this is the ability to detect issues in the first place, see #951 for one approach.

[zenmonkeykstop]

@rocodes and/or @deeplow - flagging for your attention, I think you had some notes on this.

[deeplow]

Not sure I recall my thoughts on this, unfortunately. But happy to contribute to this conversation.