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

app/models/user_preferences.rb

@@ -0,0 +1,29 @@
+DEFAULT_PREFS = YAML.load_file("#{Rails.root}/config/default_preferences.yml")
+
+class UserPreferences
+  def self.dump(value)
+    process(value).to_yaml
+  end
+
+  def self.load(string)
+    stored_prefs = YAML.load(string || "{}")
+    DEFAULT_PREFS.merge(stored_prefs).with_indifferent_access
+  end
+
+  def self.is_integer?(value)
+    value.to_i.to_s == value
+  end
+
+  def self.process(hash)
+    hash.each do |key, value|
+      if value == "true"
+        hash[key] = true
+      elsif value == "false"
+        hash[key] = false
+      elsif value.is_a?(String) && is_integer?(value)
+        hash[key] = value.to_i
+      end
+    end
+    hash.stringify_keys!.to_h
+  end
+end