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This reverts commit 6a1a4880.

This fixes #13673.
It now lists link sharing, passwords enforced, and if public uploads are
allowed.

Beside some small improvements and bug fixes this will probably the final state for OC8.

To test this you need to set up two ownCloud instances. Let's say:

URL: myPC/firstOwnCloud user: user1
URL: myPC/secondOwnCloud user: user2
Now user1 can share a file with user2 by entering the username and the URL to the second ownCloud to the share-drop-down, in this case "user2@myPC/secondOwnCloud".

The next time user2 login he will get a notification that he received a server-to-server share with the option to accept/decline it. If he accept it the share will be mounted. In both cases a event will be send back to user1 and add a notification to the activity stream that the share was accepted/declined.

If user1 decides to unshare the file again from user2 the share will automatically be removed from the second ownCloud server and user2 will see a notification in his activity stream that user1@myPC/firstOwnCloud has unshared the file/folder from him.

fixes download issue introduced by #10755

Conflicts:
	apps/files_sharing/public.php

Images on public sharing get downscaled to increase use experience - this will speed up loading time

- adding keep aspect to core/ajax/preview.php
- remove duplicate method Preview::show()
- no more hard coded mimetype of preview
- remove .png from the preview urls
- keep old route preview.png for backwards compatibility
- aspect preserving previews are now cached

includes some tests for storage providers, more to come

Create OC_JSON class, for single point of creating json responses.
No real logic change, this just cleans up the code a bit.

Created the following function:
 - checkLoggedIn
 - checkAdminUser
 - redirectToDefaultPage