drop any fallback code related to curl

Put notice about checking logs in setup warning

Fix getting the avatar of a non-existing user

Use proper error type and clean-up code

Drop example user backend

We already provide an interface for application developers, this file is outdated and thus should get removed.

Addresses No. 3 from https://github.com/owncloud/core/issues/14847

Remove OC_User_HTTP

Can also be null

If the user does not exist this returns null and can lead to nasty bugs since the IDE is not indicating this...

support paged results against 389ds.

Addresses No. 1 from https://github.com/owncloud/core/issues/14847

cron shall not operate in case we are in maintenance mode

drop any fallback code related to curl - refs https://github.com/owncloud/core/pull/14838#issuecomment-78586447

Add cURL as hard-dependency

It is required by other functionalities such as S2S anyways and ownCloud will fail hard at a lot of places without it.

we no longer support webdav locks - no need to keep the column

Do not overwrite the active menu entry of the navigation

Install command should not require table prefix

Fix the avatarcontroller cache needing the cache dir

Run `updateDataDirectory` after Update

This was removed in 82254ad5 and
unfortunatly missed by jenkins before mering into master.

Move AvatarController to AppFramework

Only make sure the cache directory exists when we use it

* Original avatarcontroller migrated to the appframework
* Added DataDisplayResponse that show data inline in the browser (used
  to retrun the image)
* Removed some unneeded code
* Added unit tests for the avatarcontroller