Show update notifications on the display

[ThomasFarstrike]

Currently, the device checks for the availability of updates and outputs the result on the serial port. But it doesn't show any update notification on the display, so most users won't be made aware of them.

This can be improved by showing a notification (like: "UPDATE ") somewhere on the display to indicate to the user that there's an update.

Some considerations:

• Screen space is scarce as the display is small, so when and where it gets displayed needs careful consideration.
• Users might not want to or be able to update for months, so this message should not be too prominent. If it gets overwritten by other text after some time, that's fine. If it only gets displayed sometimes, that's fine. If it gets displayed less frequently with time, that's fine.

Optionally, we could create a whole new "Update screen" that shows "Update is available at ." and also displays a QR code of that URL, if that makes sense. To make this work, we need a good and long-term static URL as a landing page for the updates, ideally a webpage that describes how to do the update using the web installer. This could be the web installer itself (currently https://lightningpiggy.github.io) but that URL is not under our control (github might break it in the long term) so perhaps something like https://update.lightningpiggy.com/ would be better, even if that one redirects to https://lightningpiggy.github.io for now.

[ThomasFarstrike]

This could make use of #5 but it might also make more sense to implement it in some other way.

[ThomasFarstrike]

I realized that devices might be running in the field for years before the user updates them, if they ever update at all. Especially because updating with the webinstaller requires filling the configuration (lnbits API key) again, which might be a challenge. So the update indication should be small and subtle, rather than big and annoying.

[ThomasFarstrike]

https://github.com/LightningPiggy/lightning-piggy/tree/issue-4-show-update-notifications

[ThomasFarstrike]

This is done.

From the commit message:

    Show update notification on display
    
    The notification is subtle because this might appear for
    days, weeks or months before the user decides to update,
    and we don't want to annoy them.
    
    Also, updates are probably not critical anyway.
    
    The notification is displayed by the version number (v1.9.5)
    changing to UP!1.9.5 after some time. The fact that it changes
    helps in drawing attention to it, without being annoying.
    
    Also, updates are checked only every 24 hours, as this is not
    time critical and this speeds things up and reduces load.

[ThomasFarstrike]

This is ready and deployed in v2.0.0 in the webinstaller.