Optimize duplicate queries occurring in AlertGroupFilter

[joeyorlando]

What this PR does

In AlertGroupFilter we currently have 11 duplicate queries which add ~1-2secs of unecessary request latency to GET /api/internal/v1/alertgroups calls.

The queries originate from the queryset callable arguments on several of the fields of the AlertGroupFilter class. These callables basically filter down their respective querysets to include only objects that belong to the currently authenticated user's organization.

The duplicate queries are

• 2 queries to fetch integrations
• 2 queries to fetch escalation chains
• 10 queries to fetch users

At the moment, this PR is still a draft and doesn't work as intended. It's based off of a suggestion from this discussion in the django-filter repo. See this DjangoCon talk for more context.