The process of adding default reviewers was pretty expensive. For every

default reviewer that was a match for a review request, we'd iterate

through all groups and all active and inactive users that were

specified. We then performed a lookup of the existing users and groups

to see if the new users/groups already existed and would then add them

one-by-one (which resulted in Django performing a database query

per-item to do the existence check we already did). In total, if 2

default reviewers matched, each with 2 users and 2 groups, we'd have a

total of 17 SQL queries. This would only go up as more users, groups, or

default reviewers were involved.
This change introduces new logic that minimizes the number of SQL

queries. First off, we no longer even attempt these lookups if the

review request isn't part of a repository. We then make note of any

default reviewers that matched, and fetch all their active users and

group IDs in one go, preventing any overhead for multiple matches. From

there, we add the users and groups directly to the review request in one

go without performing existence checks, instead letting Django take care

of that.
We also minimize the file lookups, returning only the raw data we need

instead of returning all data and turning them into model instances. On

top of this, we request only the attributes needed in DefaultReviewer

to perform the checks.
The result is a worst-case scenario of 9 SQL queries, covering any

number of matched default reviewers, 7 if only users or only groups

are added, and 3 if no default reviewers match.

Unit tests pass.