This commit adds most of the backend needed to implement the download
pages. It implements:

- Automatic fetch of the .iso's md5s the first time the application is
  started (and when they change), so they're available to show in the
  "Thank you" page

- Automatic fetch of the list of official mirrors for Ubuntu from the
  Launchpad API

- Download URL generation, which randomizes between mirrors to avoid
  overloading one of them