Check if app is enabled instead of if class exists

[RobinMcCorkell]

Further down in the same file, we already use isEnabled() to check if an app is enabled. This approach is better not only for consistency, but also because it avoids autoloading the class.

In addition, the existing check was bogus, since the class would get autoloaded regardless of if LDAP is enabled or not (hooray insecure autoloading!). Since the merge of #18839, that was prevented, so user management broke. This fixes #18871

@nickvergessen Hopefully you see how the other PR actually found this completely broken code, and that's a good thing?

cc @icewind1991 @DeepDiver1975 @MorrisJobke @PVince81

[scrutinizer-notifier]

A new inspection was created.

[nickvergessen]

👎 revert all that stuff please and make a proper PR

[LukasReschke]

👍

[MorrisJobke]

👎 revert all that stuff please and make a proper PR

Beside the problems with the old PR, this is nevertheless useful and also works without the changes to the autoloader.

[LukasReschke]

The autoloader PR increased ownCloud's overall security against a specific kind of attack and I'd prefer to have this in 8.2. - We can easily fix such regressions.

[oparoz]

I'm not denying the benefits of all the recent autoloading improvements, but having to work on apps while tracking core master is a major pain already and merged non-BC PRs without a heads up make it way worse, sucking bandwidth off other things which needs to get done. I'm sure it's not the last autoloading PR we'll see.

ownCloud's development process owner needs to ask the process managers to design something new so that changes belonging to that non-BC category are only merged after specific acceptance criteria have been met and if it's blocking other work, then this needs to belong to a sprint in order to be resolved quicker.

[MorrisJobke]

Tested and works 👍

This PR fixes a problem. The autoloading thingy needs to be discussed in a separate pr.