Notify users if authorization is required to proceed

Previous to this commit entering a folder without read access would show the non-descriptive error message "Could not enter folder". If the user actually is not allowed to view the contents of the folder, this is indeed true and this commit will preserve this message as is.

However, if an admin protocol is installed, users can actually view the contents of most folders after authorizing themselves as administrators. So this commit changes the error message in those specific circumstances to instead read "Authorization required to enter this folder." and provide a button to authorize themselves. This button is the "Act as Administrator" action.

If no admin protocol is installed, this commit has no effect.

The idea for this change came from Harald Sitter.

I receive funding for changes like this by the Next Generation Internet Initiative, which (as I understand it) will no longer provide funds for future projects like this if the current EU draft budget goes through as it is.