Allow blacklisting of modules

[chillu]

Prevent modules which are known to be broken or harmful from appearing on the site.

[robbieaverill]

It'd be good for us to maybe reduce the error reporting level so that these don't necessarily fill up our Raygun logs, while also adding a human-managed blacklist in the admin area or config so we can permanently ignore modules which we know are entirely gone/broken/harmful.

Examples of why this happens right now:

• Git repository has been deleted but Packagist entry hasn't
• Git repository is private

[robbieaverill]

Note that this just showed up in the logs:

Add-on chrisahhh/silverstripe-importer couldn't be downloaded; deleting from database.
Error message: The "https://api.github.com/repos/chrisahhh/silverstripe-importer/zipball/937be7a87a14c6cd1fd946b07a707b44a897ca8f" file could not be downloaded (HTTP/1.1 404 Not Found)

This would indicate that we don't need to worry about the cases I just mentioned.