* 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

Caches divided up into two groups: distributed and local. 'Low latency' is an
alias for local caches, while the standard `create()` call tries to get
distributed caches first, then local caches.

Memcache backend is set in `config.php`, with the keys `memcache.local` and
`memcache.distributed`. If not set, `memcache.distributed` defaults to the value
of `memcache.local`.

Doing this in the PHP code is not the right approach for multiple reasons:

1. A bug in the PHP code prevents them from being added to the response.
2. They are only added when something is served via PHP and not in other cases (that makes for example the newest IE UXSS which is not yet patched by Microsoft exploitable on ownCloud)
3. Some headers such as the Strict-Transport-Security might require custom modifications by administrators. This was not possible before and lead to buggy situations.

This pull request moves those headers out of the PHP code and adds a security check to the admin settings performed via JS.

This is required when working with stuff such as PDF.js in the files_pdfviewer application. Opt-in only.

Master change only because the stable CSP policies has a failback that allows nearly anything 🙈

First stab at the StreamResponse, see #12988

The idea is to use an interface ICallbackResponse (I'm not 100% happy with the name yet, suggestions?) that allow the response to output things in its own way, for instance stream the file using readfile

Unittests are atm lacking, plan is to

check if a mock of ICallbackResponse will be used by calling its callback (also unhappy with this name) method
Usage is:

$response = new StreamResponse('path/to/file');

rename io to output, add additional methods and handle error and not modified cases when using StreamResponse

fix indention and uppercasing, also handle forbidden cases

fix indention

fix indention

no forbidden, figuring out if a file is really readable is too complicated to get to work across OSes and streams

remove useless import

remove useless import

fix intendation

Potentially fixes https://github.com/owncloud/core/issues/14541 and https://github.com/owncloud/core/issues/14506

The TagManager->load() now returns null if the user is not authenticated
instead of failing with an error.

Now checking whether the written bytes match the number of read bytes.

This extends Test_NaturalSort to include a basic test for
\OC\NaturalSort_DefaultCollator()

X-Forwarded-Proto contains a list of protocols if ownCloud is behind multiple reverse proxies.

This is a revival of https://github.com/owncloud/core/pull/11157 using the new IRequest public API.