Better support for “write-only” backends

Some backends, like those for Algolia → or Default Content Deploy → (see ✨ Provide a Search API Backend Fixed ) do not actually support searching, only indexing. We should add some way for them to advertise that fact so we can, e.g., hide the indexes on them when creating searches.

It remains to be determined how we exactly want to implement this mechanism, and what effect it should have in the UI when an index/server is write-only.