A combination of using isset() instead of count() or strlen(), caching the
chunkify function, and replacing is_numeric() with some comparisons

This reverts commit 96a93192.

The URL was previously encoded twice which leads to getting redirected to a 404 page when the password has been entered incorrect at least once.

Testplan:

- [ ] Opening `http://localhost/core/index.php?redirect_url=%2Fcore%2Findex.php%2Fsettings%2Fadmin` redirects to the admin page when providing the correct credentials
- [ ] Opening `http://localhost/core/index.php?redirect_url=%2Fcore%2Findex.php%2Fsettings%2Fadmin` redirects to the admin page when providing the invalid credentials and then providing valid ones.
- [ ] Logging in as admin then going to the admin page and clearing the cookies and refreshing will show the login and when repeating the above test steps you're redirected correctly.

Fixes https://github.com/owncloud/core/issues/9804

Since the Storage interface isn't ready to work directly with exceptions
like Forbidden and NotFound, the DAV storage has been adapted to still
return false when expected.

Fixes https://github.com/owncloud/core/issues/13657

Can happen when trying to instantiate external storages that have
incomplete config, where the constructor throws an exception (the
exception is caught in createStorage())

IAppManager lives in OCP\App and not in OCP.

Fixes https://github.com/owncloud/core/issues/13710

disable the app code checker - some previous shipped apps cannot be installed from the appstore now :facepunch: