Add composite indexes based on existing queries/usage, ensuring partial
index prefixes are useful too.
- `is_active` filtering is set in the default `User` manager
- most of our user queries are per `organization`
- multiple cases filter by `username` or `email` (most notably schedule
related queries, given the low-level backend ical representation)
Also rework how users are fetched from DB when getting users from
schedules ical representation (which was particularly slow when regex
filtering by required permission).
Related to https://github.com/grafana/oncall-private/issues/2163
bba6eb333e