Aggiorna versioni di download

See merge request !20

- Added a few links in the index
- Removed the placeholder for the desktop download

- Renamed "ubuntu-desktop" to "desktop"
- Renamed "ubuntu-server" to "server"
- Prefer LTS for "server"
- Prefer "amd64" anywhere except for "lubuntu" and "xubuntu"

This commit completes the few bits left for the downloads backend, and
adds dummy content to the pages powered by it.

The "landing" page now features an ugly form for downloading, and the
"thank you" page shows the raw variables instead. The actual content
will be added in future commits.

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

Nuovo tema

See merge request !14

This commit generates a new secret key (used by Flask to sign cookies
and other stuff) the first time an instance is started within a data
directory. The key is not regenerated the next startups, to avoid
invalidating stuff.