Fix error where user could click cancel on file input and input would remain in whichever state it was in previously even error or success.
Importing Sales is now supported.

Previously imported sales are ignored.
Significant import performance gains.

Importing 482 Products (2 updates, 480 new) went from around 18,000ms to
1,350ms and in some cases ran even faster than that.

Importing 4,865 New Sales went from over 2 minutes to around 3,800ms
(3.8s).

This was achieved by implementing MiniProfiler to measure performance,
using .ToList() to prevent repeat SQL queries, and implementing the
BulkInsert plugin for EF.