unzip method

You can now send a zip archive an unzip it in the application easily.

# @group want to unzip the @zip_file in @the_folder and 
# overwrite the old content (@user is the admin who does this action)
@group.unzip_repo_item(
    @zip_file, 
    source_folder: @the_folder, 
    sender: @user, 
    overwrite: true
)