This makes it work better with old version of Nginx.

* now uses the proper URLGenerator interface
* add comment about DI problems

* removes OC_Helper::mb_substr_replace and OC_Helper::mb_str_replace
* keeps public interface wrapper working as expected

* ref #4774

Fixes #20648

No injection of userfolders etc. Only generic DI components
(IRootFolder) etc should be used to make sure we can also run this from
the cli

The share manager etc should not care about filtering stuff. They should
return what is asked for them.

To make sure hooks are always fired and child entries are always
cleaned. This logic is moved to the share manager.

* Updated unit tests

- Add davclient.js lib
- Add es6-promise required by that lib
- Wrote IE8 workaround lib/shim for davclient.js