Refactor user preferences, add defaults from file

* Turn prefs into a flat hash structure, since nesting is not worth the
trouble
* Add a custom serializer class for prefs
* Add a config file for defaults and merge set prefs with unset ones
* Use booleans for "true" and "false", and integers where appropriate

spec/models/user_preferences_spec.rb

@@ -0,0 +1,41 @@
+require 'rails_helper'
+
+RSpec.describe UserPreferences, type: :model do
+  let(:default_prefs) { YAML.load_file("#{Rails.root}/config/default_preferences.yml") }
+
+  describe ".load" do
+    it "provides default values when no preferences are stored yet" do
+      expect(UserPreferences.load(nil)).to eq(default_prefs)
+    end
+
+    it "provides default values for unset preferences" do
+      prefs = UserPreferences.load("lightning_notify_sats_received: xmpp")
+      expect(prefs[:lightning_notify_sats_received]).to eq("xmpp")
+      expect(prefs[:xmpp_exchange_contacts_with_invitees]).to eq(true)
+    end
+  end
+
+  describe ".process" do
+    it "turns all keys into strings" do
+      res = UserPreferences.process({ foo: "bar" })
+      expect(res[:foo]).to be(nil)
+      expect(res['foo']).to eq("bar")
+    end
+
+    it "converts value 'true' to boolean" do
+      res = UserPreferences.process({ lightning_notify_sats_received: "true" })
+      expect(res['lightning_notify_sats_received']).to be(true)
+    end
+
+    it "converts value 'false' to boolean" do
+      res = UserPreferences.process({ lightning_notify_sats_received: "false" })
+      expect(res['lightning_notify_sats_received']).to be(false)
+    end
+
+    it "converts value string with integer into integer" do
+      res = UserPreferences.process({ lightning_notify_sats_received_threshold: 1000 })
+      expect(res['lightning_notify_sats_received_threshold']).to be_a(Integer)
+      expect(res['lightning_notify_sats_received_threshold']).to eq(1000)
+    end
+  end
+end