Add db indexes to user table

[matiasb]

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