Added support for extra backends

Somebody had forgotten "OC_User::setupBackends();"...
Converts em to px values for styles in Files.
Cleans up Core apps for relative(em) to absolute(px) styles.
All Core Styles from em->px, apart from styles.css
Added ownCloud backend for external storage

To make it possible to use the short ownCloud URL (without specifying
webdav.php or remote.php), a new backend is available for ownCloud.

The user must specify the host + context path in the "Url" field (which
is mapped to the "host" parameter) and the subdir to mount in the "Root"
field.

This is to prevent confusion because some users forget to append
webdav.php or remote.php to the WebDAV URL.

Fixes #4923
No decimal points for Kilobyte and Byte #5371.
Symfonys addCollection() with multiple arguments is deprecated, fix deprecation warning
#6051 [ux][files] Move File Spinner

Added spinner to file icon when moving to new folder or out of folder.

Commit is related to #6051
Fixed indention.

As per @kabum request, indention on line 694 has been fixed.
Fixed Dropbox/Google storage async save call

When clicking "Grant access", the settings for Dropbox/Google were saved
through a call that gets cancelled when redirecting to the grant page
(for example in Firefox)

This fix makes sure the "save settings" call finished before redirecting
to the grant page.

Fixes #6176
change mail address by pressing enter - fixes #6179
correctly mark app management active
fix typo
Moved content disposition code+workarounds to OCP\Response

Added new OC\Response API called setContentDispositionHeader() that
contains the needed workarounds for UTF8 and IE.

Refactored download code to use the new API.

Removed unused trashbin download file.
Added workaround for Android content disposition

Fixes #5807
Fixing Issue #6301 on master branch
Added private var $certPath to store the user root cert

Move logic to determine the $certPath path to the constructor and modify to get the path from OC_User::getHome()

Add curl options to use the certificate to avoid certificate errors with self-signed certicates in the downdload/upload method so we don't get blank files
Fixing tab/space coding style
Only set SSL things when ```$this->secure``` is ```true```
Fix ```createCertificateBundle``` and  ```getCertificates``` methods
from ```OC_Mount_config``` to use ```OC_User::getHome``` to get the
path
fall back to getLocalFile if storage doesn't support fseek
fix the config option to remove the ability for users to set their displayname