Basic donation records

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

app/models/donation.rb

@@ -0,0 +1,10 @@
+class Donation < ApplicationRecord
+  # Relations
+  belongs_to :user
+
+  # Validations
+  validates_presence_of :amount_sats
+
+  # Hooks
+  # TODO before_create :store_fiat_value
+end

app/models/user.rb

@@ -3,6 +3,7 @@ class User < ApplicationRecord
 
   # Relations
   has_many :invitations, dependent: :destroy
+  has_many :donations
 
   validates_uniqueness_of :cn
   validates_length_of :cn, :minimum => 3