Support loading of RFID data via CSV

[cjbarth]

Building upon the concept discussed in #941 for exporting data via CSV, it would be nice if RFID data could also be imported via CSV. However, this may not be entirely needed if #940 allowed for sharing of RFID data across a power sharing group, not just for metering but for access as well. Still, in the case where some EVSE may not be for all to use, it would be valuable to be able to import the CSV to each EVSE individually.