Describe the bug:
When spaces are disabled, navigating to the detections page will throw errors.

Steps to reproduce:

1. Disable spaces: xpack.spaces.enabled: false in kibana.yml
2. Navigate to the detections page
3. Observe toaster errors, network errors, and

Expected behavior:
If spaces are disabled, detections references the index specified in xpack.siem.signalsIndex

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:
This was discovered as part of a refactor: 00122ec