WIP: various fixes

• move root-password and account steps to the end of wizard flow (re #3)
• limit possible password characters to whitelist of most printable ASCII characters (re #10)
• validate username according to https://systemd.io/USER_NAMES/#common-core (re #9 – TODO: "Explain the rules")
• don't crash on command errors, instead display warning with error output (re #8 – TODO: "handle some common errors gracefully" (which?))
• sort timezone selections alphabetically (re #2)
• make timezone selections searchable (re #2 – TODO: default to Europe/Berlin)