Bring back disable app safeguard

[juliusknorr]

It seems that the safeguard to disable apps if the instance is not working anymore after an upgrade is no longer being used anywhere.

https://github.com/owncloud/core/pull/17451/files#diff-556b4fe66666d9c7f9478b16e42d65f3b11c47e11a3e34f44467aaabe4dbded9

The backend bits are still there:

server/lib/base.php

Line 1006 in 052dcde

if ($requestPath === '/disableapp'

Maybe we should reintroduce the check in the vue based frontend.

[susnux]

The endpoint was removed here: f2c36a5

Reasoning was that its a bit arbitrary to check only the files app and more important nowadays app loading cannot break the app boot process anymore.