Fixed share with link checkbox missing as regular user

Instead of loading the app config setting "shareapi_allow_links" using a
synchronous ajax call that fails when the user is not an admin, this fix
puts the flag directly in the template so it doesn't need to be loaded
afterwards.

Fixes #5440
Merge pull request #5441 from owncloud/files-sharewithlinkasnormaluserfix

Fixed share with link checkbox missing as regular user