Add position to Location model and acts_as_list
Make sure Location position defaults to 0.
Update Location position on drop.

Current issue: when two Locations end up with the same position, sorting is not
preserved. I must have solved this with Triplocations, so I need to just figure
out how.
Remove Locations render on sync to prevent sorting from skewing order.
Add ordering to a User's positions so they are accurate when server-rendered.
Merge branch 'sortable_locations' into development