Basic donation records

Adds donation model/table and basic manual management in the admin
panel, as well as basic listing of users' own donations.

spec/models/donation_spec.rb

@@ -0,0 +1,5 @@
+require 'rails_helper'
+
+RSpec.describe Donation, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end