Return plausible isReadable() default impl for ext storage

Also adjusted all ext storage backends to not override these when the
default behavior is expected.

When an ext storage doesn't implement isReadable(), always returning
true made the file scanner believe that the file exists and creates a
cache entry with the size zero.

This fix makes the default impl of isReadable() use file_exists().

Fixes #5940

This reverts commit 3488f0b7.

Incognito mode

set incognito mode for public.php calls. Because in this case ownCloud should always work the same way as if no user is logged in

add incognito mode, allows to hide my user ID. For example, this is useful to access public resources while a user is still logged in

Movie preview - use frame after 5 seconds

use relative paths to create images from a file. 

Add public API for \OC\Avatar

no user is logged in (e.g. public gallery share)

in some cases 1MB was no enough to generate thumbnail

Fixed path normalization to prevent dot dirs

Make working en_US.UTF-8 locale a hard requirement

Fixes #5945 where stat(.) would cause the backend OC to cache a dot dir
(only in older versions)

Quota storage wrapper is now used for all users in sharing mode

Added min width to files table

Fixing l10n again master

Having a too small widths for the files table doesn't make sense as it
overlaps file actions with the files themselves and make them
unclickable, especially when viewed on mobile phone.

The compromise that this fix introduces is to have a minimum width that
will trigger horizontal scrolling, which will at least make it possible
to click files on mobile phones.

LDAP Wizard: add objectclass for group detection in Zimbra

Public view fixes

To find out whether to apply a quota, we now try and detect whether the
storage to wrap is a "Home" storage.

When accessing a shared folder, the folder's owner appears as mountpoint
but wasn't wrapped by a quota storage wrapper.

This fix makes sure that all home storages are wrapped by a quota
storage wrapper, if applicable, to make sure quotas are respected when
uploading into shared folders.

Fixed error-checking error in sftp.php

Fixed move operation to pass the whole URL as expected