Default global searches to Advanced search when Elasticsearch integration is enabled

One of the primary benefits to enabling Elasticsearch on GitLab.com is that we enable cross project code search. Today, a "global search" (not project or group scoped) is defaulting to "basic search" since the entire instance is not indexed.

In order to provide the most benefit of inner-sourcing across GitLab.com, we should default search to "advanced search" across all indexed groups. This should provide a positive experience for users looking for other projects and examples on GitLab.com. The downside is that they will only be able to search for things that are in the index. If a user wants to search in a specific group or project, the filters allow that and will direct the user to "basic search" if the group or project is not indexed.

This will also impact self-managed customers using limited indexing.