Fix bug in not changing location/agency

There was a bug where the location wouldn't change despite selecting
a new location. This only occurred after you selected a location
for the first time.  Force the delete for the existing tag, and
then store the new tag.

Addresses https://github.com/USStateDept/midas-crowdwork/issues/28
Always use latest location/agency tags

In the rare event that multiple tags are stored, always
use the latest stored value

Addresses https://github.com/USStateDept/midas-crowdwork/issues/28