Add concurrency limit to Advanced Search

Problem to solve

Currently, Advanced Search could potentially overload an instance if too many repository indexing jobs are scheduled.

Proposal

I believe that we need to introduce a new setting to allow specifying the number of repository indexing jobs that can run concurrently.

Release notes

We've added code indexing concurrency to Advanced Search admin setting, which allows you to limit the number of concurrent code indexing background jobs. Before this change you could only do it by setting up dedicated Sidekiq processes.