'Urgent message' feature

[Tim]

Some urgent temporary messages need to be transmitted to members, but not necessarily by email.
Example 1: Ask members to enroll on the planning, when the planning is empty
Example 2: A last-minute delivery is expected

Must have

• Popup showing the message when a member logs in the app
• Message is editable from the admin interface
• Message is active for the current day only
• Message should be tweet-like (~142 characters, keep it short)