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` action begins 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.