Add configuration option for member deletion rate limit

Proposal

We made a change that introduced a rate limit for member deletion events using the API, in order to mitigate a Sidekiq performance issue. This has negatively affected at least one self-managed customer's (GitLab internal) workflow, as they're now getting failures for automated member deletions using the API. Since they're self-managed, I believe that we should be able to introduce a configuration option to either disable or adjust this rate limit.

If we cannot add configuration options for this rate limit, I propose that we add more information to the relevant documentation with an explanation, and link back to this issue.

Workarounds

The only workaround I can see here is to add a wait to any operation that's exceeding this rate limit. However, given the way the customer is using this, introducing a wait is cumbersome and noticeably slows down processes that were previously very quick.

Implementation guide

This should be straightforward to add a new columm, then use the new column to be configurable.

We then expose the new column to be editable in the admin UI. Grep for issues_create_limit to see how it is done in the past.