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/views/settings/_lightning.html.erb

@@ -8,13 +8,11 @@
       description: "Notify me when sats are sent to my Lightning Address"
     ) do %>
       <% f.fields_for :preferences do |p| %>
-        <% p.fields_for :lightning do |l| %>
-          <%= l.select :notify_sats_received, options_for_select([
-            ["off", "off"],
-            ["Chat (Jabber)", "xmpp"],
-            ["E-Mail", "email"]
-          ], selected: @user.preferences.dig('lightning', 'notify_sats_received')) %>
-        <% end %>
+        <%= p.select :lightning_notify_sats_received, options_for_select([
+          ["off", "disabled"],
+          ["Chat (Jabber)", "xmpp"],
+          ["E-Mail", "email"]
+        ], selected: @user.preferences[:lightning_notify_sats_received]) %>
       <% end %>
     <% end %>
   </ul>

app/views/settings/_xmpp.html.erb

@@ -3,8 +3,8 @@
   <h3>Contacts</h3>
   <ul role="list">
     <%= render FormElements::FieldsetToggleComponent.new(
-      field_name: "user[preferences][xmpp][exchange_contacts_with_invitees]",
-      enabled: @user.pref_enabled?("xmpp:exchange_contacts_with_invitees"),
+      field_name: "user[preferences][xmpp_exchange_contacts_with_invitees]",
+      enabled: @user.preferences[:xmpp_exchange_contacts_with_invitees],
       title: "Exchange contacts when invited user signs up",
       description: "Add each others contacts, so you can chat with them immediately"
     ) %>