The setting keys should not be prefixed by commerce_file

Problem/Motivation

There's no reason for our settings to be prefixed by commerce_file_enable_download_limit, since the config object is already commerce_file.settings.

Let's rename "commerce_file_enable_download_limit" to "enable_download_limit" and "commerce_file_download_limit" to just "download_limit".

Now that we're introducing download limits per variations, I'm actually wondering whether we should get rid of the global settings.

I'm afraid that having both will create confusions... For example, how do you actually set an unlimited download limit for a given variation if a global limit is configured?

Should you set the limit to 0?

Since the module doesn't have a stable release, we can probably go ahead and rename the settings for now although it probably doesn't hurt to write a quick update to migrate to the new settings keys.