[Downloads] Make the download button actually work

Right now the download button does nothing.

There's a route for it right now,

Download:
    pattern:  /download/{type}
    defaults: { _controller: "Application:Index:download"}

but there's no backend controller code.

We need to

Change {type} to {fileID} in the route
Take in the fileID in the controller action
Make a call to DownloadsHelper (downloads.helper service) and pull out the Entity/DownloadItem.php entity and grab the filename from it
Make an insert into the download_entry table with the download_item_id so we basically track it.
Grab the file from perhaps /public/downloads/ and fire it back to the user using the standard Content-Disposition: attachment header
If the user chose the "with vendor" option then we str_replace ".zip" with "-with-vendors.zip" and that will match the equivelant file.
Redirect the user to a "Thanks for downloading page" with a link to 'installation instructions' page which is just linking to the documentation on Getting Started for now.

ppi / website