check if encryption app is enabled before trying to calculate file size
check if key exists before reading it
fix check if app is enabled

Conflicts:
	apps/files_encryption/lib/proxy.php
Use display name from magic cookie, fixes #4884
Merge pull request #5415 from owncloud/fix_4884

Use display name from magic cookie, fixes #4884
Merge branch 'stable5' into ocs_share_api_stable5