update_pages_service.rb

RubyZip allows us to perform strong validation of
expanded paths where we do extract file.

We introduce the following additional checks
to extract routines:

1. None of path components can be symlinked,
2. We drop privileges support for directories,
3. Symlink source needs to point within the target directory,
   like `public/`,
4. The symlink source needs to exist ahead of time.