* Add host count to software API
* Update docs
* Update fleetctl tests to account for host counts
* Update docs to mention host_count special case
* Update func comment
- Include only hosts that the user has access to in search targets API.
- Add parameter to specify whether `observer` hosts should be included.
- Generate counts based on which hosts user can access.
- Update API doc.
- Maintain software inventory with detail queries.
- Associated database migrations.
- Feature flagged off by default (see documentation for details to turn on).
- Documentation.
- New test helper for slice element comparisons skipping ID.
