Customizable Message Subjects

[glenn-sorrentino]

Motivation Customers have reported that when they receive whistleblower messages, they come in waves, and they currently have no way of triaging information. Messages can be for anything, including:

• Asking for 1:1 help
• Requesting the person for a speaking engagement
• Asking for a referral
• Other stuff

Acceptance Criteria

• For paid users and Personal Server users, they may create up to 5 customizable subject lines for the Pro/Personal Server tier, and 20 for business.
• A user will navigate to Settings to enable the feature
• The UI exists in the "Email & Encryption" tab
• Adding a custom subject creates a new select menu on the submit_message page.
  • If no custom subject lines are created, the UI in the page does not show the Subject dropdown.
• The settings UI is a simple text input. Adding a new subject adds an entry to a list under the input for visual confirmation.
• The user can edit or delete the subject lines they've added.
• The subjects should be unencrypted so the tip line owner can quickly get a glance of their messages without being required to decrypt them individually, especially from a mobile device.

[glenn-sorrentino]

Depends on #364