Reload the files app in case of authentication errors, expired tokens or disabled app

Reloading will triger the full server side handeling of those errors

formatting

fix missing semicolon + some jshint warnings

OC_Util::setupFS($user) will create a data dir for the given string - no matter if the user really exists - OCP\JSON::checkUserExists($owner); introduces a ready to use check which will bail out with an JSON error

- L10N now converted to string to make them work with json_encode
- Added specific error message when server doesn't allow fopen on URLs
- Fixed client side to correctly show error message in a notification
- Added OCP\JSON::encode() method to encode JSON with support for the
  OC_L10N_String values

Conflicts:
	lib/private/vcategories.php

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

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 :)

Admins can still enable it via config.php