Advanced Premium configuration

[echarlie]

probably create a table in donations/models.py for premiums:

basically Premium_bool and Premium_size with appropriate fields, and the control panel allows setting 0) item type (bool or sized) 1) minimum donation, 2) name (which we sanitise into a slug for the DB), 3) size options (comma-seperated, converted into selection)

this should make #18 and #20 simpler to implement

[echarlie]

this doesn't handle the case where we offer multiple T-shirt colours, or offer T-shirt cuts. Continue speculating on the ideal model here...