Add maintenance mode setting with exempt accounts and customizable messaging

## Summary

Add a maintenance mode feature configurable from the admin WebUI settings page.

## Requirements

- Admin can enable/disable maintenance mode from the WebUI settings page.
- When enabled, regular users see a maintenance page instead of the normal app.
- Admin can select exempt accounts; those users continue to see the normal app instead of the maintenance page.
- The maintenance page's title and messages are customizable by the admin.

JIRA Issue: FR-3233

Start at the admin WebUI settings page and trace how settings affect the normal application view. Define the maintenance page, account exemptions, and customizable title and messages from the requirements; done means admins can configure the setting and regular and exempt users see the expected views.

Written by the indexing model from the issue text.