Merge pull request #7617 from nishiki/cache_user

Cache user
Merge pull request #8009 from owncloud/feature/add-sharing-target

Add option to getUsersSharingFile() to get the paths for the shared users
Fix error when viewing expired link
add some action items
first steps to remove the shared folder:

- mount shares to the root folder instead of "Shared/"
- navigate in shared folder and sub-folders
- show previews
- show correct file permissions
- download/edit files
make sure that we have the right permissions for the mount points
no exception for the root of the mount point (formerly Shared/), just get the file cache information from the cache
allow to remove and change mount points
user should be able to rename/delete shared files if the owner allowed it
fix getMimeType call, we always need to check the source path
add api to get shares from a specific user
generate unique target name
show "shared by ..." for share mount point
don't overwrite shared folder icon
we no longer need to handle the Shared folder different from any other folder
fix share api tests
fix target generation for group shares
fix encryption tests after the removal of the shared folder
fix updating of shared files
fix deleting of shared files