Rename public facing Yokozuna to Riak Search 2.0

We need to rename all of the cases where the term yokozuna is public facing, with some variety of `search`, or `rs`, so as to clarify that this is a Riak Search 2.0 project.

- [ ] Rebrand old RiakSearch config (etc?) to `legacy_search`
- [x] Update HTTP admin resource names from `yz` prefix to `rs`
- [x] Update all yokozuna.schema cuttlefish mappings from `yokozuna` prefix to `search`
- [ ] Rename `yz_index` bucket label to `search_index`, via a new `YokozunaAssociateIndexPbc` request
- [x] Update docs with use new resource paths and settings