fix error message, add "appid" to the output to know which app caused the error if OC_App::getStorage() fails.
Port of approved patch for stable45: https://github.com/owncloud/core/pull/1222

refs #1162

Oneliners are ugly.

This is necessary to enable the desktop sync client to react properly.
Currently the SabreDAV plugin OC_Connector_Sabre_MaintenancePlugin is not executed because this error page is returned before the SabreDAV code is executed

Let's remove it, it's also not secure anymore with the introduction of
our API etc...
(And doesn't work with ldap etc…)

Thx @Raydiation

There was no "isAdminUser()" function which returned bool. This is
irritiating as there were a loooooooot of places in the code which
checked this itself with `OC_Group::inGroup($uid, 'admin)` - why not
use a function for this?
(Especially if you consider that we might change the group name in the
future, which would lead to problems then)

Additionally, @Raydiation needed such a method for his AppFramework :)

otherwise /foo/bar would be detected as a subfolder of /bar

THX @icewind1991

isSubDirectory() checks if a specified $sub is a subdirectory of the
$parent, this is needed to prevent file inclusions.

Actually, the current code is more kind of a "hack" which I always
struggle over if browsing through source. So this should be a much
better implementation.

The implementation is really straightforward:
- [realpath()](http://php.net/manual/function.realpath.php) expands all
symbolic links and resolves references to '/./', '/../' and extra '/'
characters in the input path and return the canonicalized absolute
pathname.
- [strpos()](php.net/manual/function.strpos.php) returns FALSE if the
substring wasn't found.

Since this is an absolutely critical piece of code, I'd like to ensure
that this is absolutely safe!

webdav returns "503 Service Unavailable" in case the ownCloud server is currently in maintenance mode.

This means that OC_Connector_Sabre_Client is no longer needed

added cleanup recommendations by including ocs in the cachedKey method and moving the ocs routes stuff at the end of the loadRoutes method

fixed error that displayed a white page and a warning that routes could not be loaded from apps that was caused by the ocs_api merge