Add option to disable in-app update

[maltfield]

Is your feature request related to a problem? Please describe.
The BusKill app needs a a way to disable the in-app update functionality

Describe the solution you'd like
A boolean option in the buskill config file, when set, should:

1. Prevent the "Update" button from even being visible in the navigation drawer
2. Cause any of the upgrade()-related functions to WARN and then immediately return

Describe alternatives you've considered
A debian patch

Additional context
This is needed when BusKill is installed by a third party (secure) package manager, such as Debian's apt.

For more info, see:

• Support for installing in apt (.deb) #31 (comment)

[maltfield]

This is currently blocked by #16

[fmarier]

Here's the temporary patch in the Debian package: https://salsa.debian.org/debian/buskill/-/commit/3907ca10c431bf31c66ac91d5b17335d38d975eb