Replace `pt-os-updater` with web portal's OS updater

[m-roberts]

Currently the OS updater uses a systemd service that runs on each boot. It checks /etc/pi-top/pt-os-updater/last_checked_date to see if a day has passed, before deciding if the user should receive a notification that contains the action command to start the update in it.

This new system should be robust and easy to test with confidence. Therefore, I propose for a first version:

• Web portal notifies user if there's an update, says to start Updates from the Start Menu
• .desktop file runs web-renderer at update page
• Updater window can be closed and update will continue

Notably, we drop the config file for 'last checked date' until we build a better system for the web portal to handle how it checks for updates. This is likely to grow to encompass many different use cases:

• Schools don't want to update and grind their networks to a halt - they need a way to bypass regular checks
• Some users may want to auto-download in the background and upgrade on shutdown

Some thought should be given to the scope of the use-cases and an appropriate configuration pattern should be chosen.

Possibly we can consider some of the following:

• Add an action to open web-renderer at update page directly from notification
• Detect if user closes window, use notification to confirm
• Window opens without option to close except for from the UI itself
• IPC commands (e.g. web-portal-ctl update-window)?

[m-roberts]

Note that this needs to have identical UX to the previous OS updater - particularly use of:

/tmp/pt-firmware-updater.ready
/tmp/pt-firmware-updater.extend-timeout