Bug 1093828 - Add a site wide banner to warn users that the database is read-only

This really should return a boolean. If it returns `{"DB_READ_ONLY": "OFF"}` and you for some reason do:
```
{% if DB_READ_ONLY %}
<h3>Oh noes!</h3>
```
It will really print out `<h3>Oh noes!</h3>` because `'OFF'` is a truthy value. 
Also, if you do that you don't need the comment on the line above.