Hide buttons for disabled features

Previously we had a gin@home fork and container image that was modified for on-premise deployments of GIN. The modifications disabled features and UI elements that require extra (micro)services that we run ourselves:
- Data search (gindex)
- Dataset publication (gin-doi)

Maintaining a fork with these changes is cumbersome. Instead we should hide the UI elements (e.g., the Request DOI button) when the service is not configured.

Start by locating the UI elements for data search and dataset publication, including the Request DOI button, and then trace how service configuration is exposed to the frontend. The work is done when elements requiring unavailable services are hidden while configured services retain their existing behavior.

Written by the indexing model from the issue text.