Index Range lost when index is closed

### Problem description

When an index is closed, there is no index range available anymore afterwards. This makes it hard for users to determine, which "historic" index they need to reopen to have data for a specific time range available again.

A proposed solution would be to merge old index ranges into newly calculated ones for closed indices, to keep old index ranges for closed indices (which should not change anymore) around.
### Steps to reproduce the problem
1. Go to indices page on web interface
2. Close an arbitrary index
3. See that "Time range of index is unknown, because index range is not available. Please recalculate index ranges manually."

Optional:
4. Click Maintenance -> Recalculate Index Ranges
5. See that nothing changes
### Environment
- Graylog Version: Graylog 2.0.0-beta.2-SNAPSHOT (9c0846cf71b96735239488523d8f65863c219145e)
- Elasticsearch Version:
- MongoDB Version:
- Operating System:
- Browser version: