The code for search indexing was old and lacked sufficient unit tests, and this led to some missed performance issues when indexing content. First of all, we had previously had an N+4 issue when indexing users without a profile, due to an issue where `User.get_profile()` wasn't respecting a `None` profile returned from a `select_related()`. This fix has been rolled up into its own change, but is notable as it's part of this work. Our main queries used to return results for indexing weren't using modern Django ORM capabilities to fetch related results. We were stuffing a lot of objects into `select_related()`, when `prefetch_related()` would have been more appropriate. If 20 items all referenced the same repository (for example), we'd be re-fetching that repository information as part of each review request query, instead of batching the repository IDs and fetching only unique ones, which `prefetch_related()` gives us. We now have more queries, but smaller ones that can better utilize indexes. The prefetches make use of `Prefetch()` and `only()`, which lets us gather just the fields we need on each object, instead of fetching every column. This means smaller results and less processing. We use this in every prefetch. `only()` is also used to limit the fields on the main `User` and `ReviewRequest` objects we're indexing. We now only fetch the fields we know we'll need for indexing or for computing data for indexing. This also covers any `select_related()` needed on the objects. `LocalSite`-related queries is only included when we know the server has Local Sites. In the typical case, this means fewer, slimmer queries and less processing time. There is a caveat in that if a server goes from no Local Sites to 1+ Local Sites while indexing is occurring, various ACL logic may end up fetching associated Local Sites as independent queries while processing ACLs and such, but this wouldn't happen at the next re-index, and is pretty unlikely to pose a problem in real usage. Most importantly, we now have extensive unit tests for the indexing logic that asserts queries, making sure we don't have any N+1 issues now or later as logic evolves. This will also help us keep things manageable as we add new indexing improvements. To keep all this logic maintainable and to further avoid errors, typing and modern docstrings have been added.