Add "data export" feature

- Adds a button in Account Settings where you can request a ZIP export of your
  Fizzy data
- Export files are created in the background. When ready, a link to
  download them is sent to the requester.
- Exports expire after 24 hours. And are limited to 10 per day.

config/recurring.yml

@@ -27,6 +27,9 @@ production: &production
   cleanup_magic_links:
     command: "MagicLink.cleanup"
     schedule: every 4 hours
+  cleanup_exports:
+    command: "Account::Export.cleanup"
+    schedule: every hour at minute 20
 
 <% if Fizzy.saas? %>
   # Metrics