Tails Installer should autodetect if Tails is installed on the destination device

To simplify the workflow regarding install or upgrade (and as long as Tails Installer does the upgrades, see #7499) we propose the following:

• Tails Installer start from its main interface straight away (see #8859 (closed)).
• When the destination device has Tails installed already, the “Install” button is rename “Reinstall” (or something similar) and an “Upgrade” button appears below it.

Feature Branch: feature/8860-tails-installer-improvements installer:master,tails/master

Attachments

• 
• install-upgrade-switching

Parent Task: #9005 (closed)

Related issues

• Related to #8862
• Blocks #13424 (closed)
• Blocked by #14695 (closed)

Original created by @sajolida on 8860 (Redmine)