Sharing improvements

php upload errors are written to log

Changed access mode for Dropbox to 'auto'

Dont scale up the preview for publicly shared images

point should be adjusted to ne unique again

allow admin to disable sharing for specific groups of users

Priority on external storage mounts

Handle excpetions in console master

Add a method to get the values for multiple users to OC\Preferences

A mount applicable to all users is as good a definition of global as can be

Each storage backend has a default priority, assigned to any system mounts
created in ownCloud. mount.json can be manually modified to change these
priorities.

The priority order is as follows:
 * Personal
 * User
 * Group
 * Global

Within each mount type, the mount with the highest priority is active.

The storage backend defaults were chosen to be the following:
 * Local - 150
 * Remote storage - 100
 * SMB / CIFS with OC login - 90

[sharing] move the mount point up if the parent folder no longer exists

rotate custom logfile if defined

[master] Fixed issues with renaming

[master] Fixed for public link page

[encryption] improved error handling during password change