[xenial] Implement securedrop-admin os-release-upgrade feature

[eloquence]

Description

As part of #3204, we may want to implement an os-release-upgrade feature for securedrop-admin. This would be run on the Admin Workstation, and guide the administrator through the upgrade process from Trusty to Xenial.

This tool may offer to perform a backup/restore, and perform the do-release-upgrade step as well.

Depending on resolution to #3968, we may be able to eliminate prompts caused by the upgrade itself -- or at least provide some in-tool guidance.

User Stories

As an administrator, I would like to have some guardrails for the system upgrade, so that the room for error on my part is reduced.

[eloquence]

For now we've decided against this, since the upgrade is unlikely to be non-interactive, and will lean heavily on docs.