Basic donation records

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

db/migrate/20201130132533_create_invitations.rb

@@ -8,6 +8,7 @@ class CreateInvitations < ActiveRecord::Migration[6.0]
 
       t.timestamps
     end
+
     add_index :invitations, :user_id
     add_index :invitations, :invited_user_id
   end

db/migrate/20201217161544_create_donations.rb

@@ -0,0 +1,15 @@
+class CreateDonations < ActiveRecord::Migration[6.0]
+  def change
+    create_table :donations do |t|
+      t.integer :user_id
+      t.integer :amount_sats
+      t.integer :amount_eur
+      t.integer :amount_usd
+      t.string :public_name
+
+      t.timestamps
+    end
+
+    add_index :donations, :user_id
+  end
+end