Change default number of index partitions (UI ONLY) to 1

New indexes created in the UI should default to 1 partition.

The tooltip for index partitions needs to be updated.  Proposed text:

"Partitioning the index allows multiple nodes running the search service to share the indexing and query workload.  In some configurations this can reduce index build time and improve query throughput.  For more information about choosing the correct number of partitions see the documentation." (with appropriate link)