MemberSearch cleanup and polish

As a follow up to #64, this renames `GitHub::Ldap::Members` *member search* strategy to `GitHub::Ldap::MemberSearch` and aims to finish up any significant missing pieces:

- [x] configuration
- [x] Active Directory strategy (followup?)
- [x] detect strategy

cc @jch 

@jch this was based on the existing tests for the `MembershipValidator::ActiveDirectory` strategy which does this same stubbed vs integration tests, introduced in #61.

But you're right, I think the value is in that we can have CI help us at least make sure superficial regressions are avoided, and when real development work is happening we can run the local integration tests against AD to validate the changes. This was my method when writing this strategy, and it made the development smooth and quick!