Added possibility to use Drill 2 online instance as offline app

This simple change allows to use HTML5 AppCache for online-hosted Drill 2 instance. This makes it possible to use it without Internet connection.

If any external resources are added or updated, they have to be included in cache manifest. If Drill 2 resources are changed (but there are no changes to their names), manifest revision number must be bumped (any changes to manifest file trigger client-side cache update).

**Note:** I'm not sure whether this would affect external resources in questions (images embedded using Markdown). It may be necessary to add
```
NETWORK:
*
```
to AppCache manifest file if external images are visible only on the first page load and disappear on subsequent reloads.