Flush and re-put mapping on admin sync request

This fixes #46 and #63.

Please be aware that this will flush each index and put the mapping prior to any post being indexed when the admin `Start Sync` sync happens.

If you're not comfortable with this approach let me know and we can discuss it further. 

Tested locally and it is working with no issues on single site.

@tlovett1 #review and #merge if comfortable.