search contexts: improve validation and create/update perf

Created by: novoselrok

See https://github.com/sourcegraph/sourcegraph/pull/26588 for first attempt

Instead of validating and retrieving the ID for each repository on its own, we can bulk request the data for all repositories and validate them all at once. This will speed up the client UX of validating and creating/updating search contexts.

We can do a similar improvement on the backend where we extract all repositories in a single query.