Option to disable self upgrade

[makuhama]

This is a follow-up to #3297. With the help of @mahagr I succeeded in packaging Grav for Arch Linux. The package already contains a patched GPM that doesn't allow gpm selfupgrade. But that is only half the Story. I also need to prevent upgrading Grav via admin plugin. I was planning to also deliver a package for the admin plugin - patched in a way to disable upgrading Grav from the admin plugin. But here it gets nasty:

1. The admin plugin is dependent on three other plugins. So I have to bundle and watch all four plugins (admin + 3 dependencies) and deliver a new package any time one is released.
2. The patched admin plugin also needs to prevent upgrading itself and the 3 dependencies - for obvious reasons. Haven't investigated yet whether this is doable and how much effort.

So I'm kindly asking for an option that prevents any self upgrade of Grav - be it via gpm or via admin plugin.

I know this is a very special request for 2 or 3 packagers worlwide :-)

I'm aware that people still can upgrade manually (downloading ZIP files and copying around stuff in the filesystem). They can also manually toggle the above mentioned option. But IMHO then these people are completely on their own.

[mahagr]

Upgrading plugins and Grav will already be disabled if you use symlinks. So if the plugins are outside of the Grav root, but symlinked to the site, updating them will be prevented. This includes manual upgrades, though.. you can always remove symlink manually and to the upgrade in that way.