This reverts commit 6a1a4880.

Nobody reads the warnings anyways and so we should enforce it at installation time... Also allows us to get rid of some duplicated code.

To test change the `default_charset` to something other than `utf-8` or `UTF-8`, both should work fine with that change here. An error should then get shown.

We already set those default charsets in the shipped .user.ini and .htaccess

Fixes new avatar selection in master half, other half will work when https://github.com/owncloud/core/pull/14266 has get merged.

Shocking to see how much places in our code do it wrong 

APCu before 4.0.6 is unbelievable buggy and tend to segfault the PHP process (i.e. the whole webserver)

This potentially fixes https://github.com/owncloud/core/issues/14175

Requires a backport to stable8

This changeset removes the static class `OC_Request` and moves the functions either into `IRequest` which is accessible via `\OC::$server::->getRequest()` or into a separated `TrustedDomainHelper` class for some helper methods which should not be publicly exposed.

This changes only internal methods and nothing on the public API. Some public functions in `util.php` have been deprecated though in favour of the new non-static functions.

Unfortunately some part of this code uses things like `__DIR__` and thus is not completely unit-testable. Where tests where possible they ahve been added though.

Fixes https://github.com/owncloud/core/issues/13976 which was requested in https://github.com/owncloud/core/pull/13973#issuecomment-73492969

This enforces proper types on POST and GET arguments where I considered it sensible. I didn't update some as I don't know what kind of values they would support 

Fixes https://github.com/owncloud/core/issues/14196 for core

Orthography: "Back-end" -> Backend (without hyphen) and correction of "users" to the genitive "user's"

Added the missing space between value and unit, as it is formatted in all other places across ownCloud.