Implement transparent support for TUF

[doanac]

Add primitive and "invisible" support for TUF. This will include removing the distinction between "production" and "CI" devices. If a user chooses to do advanced TUF operations, such as multi-signatures, we won't stop them.

When you upload a build to our server (create an update), the backend signs it with an online Targets key

We'll also introduce periodic logic to refresh timestamps/snapshots. Relates to #21

[doanac]

I have a branch I'm experimenting on: https://github.com/foundriesio/update-server/pull/new/tuf-support

I'll start moving this to a PR once the code it depends on (PR #184) gets merged

[doanac]

This issue will be fixed by PR #191