Add ability to disable uploads of media type while retaining existing media of that type

[ayleph]

Per a discussion in #382, I think it would be nice to be able to disable further uploads of a particular media type without having to delete all the existing media of that type. For example, let's suppose I run an instance with multiple users and people have been posting awesome cat videos. Eventually I get a notice that my VPS is running low on disk space, and I want to disable further video uploads. But I don't want to delete the existing videos!

If I were to disable the video plugin entirely, then existing videos would fail to load or lead to crashes, based on the traceback reported in #382. So instead, I'd like to have some kind of per-plugin directive that lets me keep the media type enabled while disabling future uploads.

I'm thinking something like below.

 [plugin_spec]
+# Prevent users from uploading new media of this type?
+disable_upload = boolean(default=False)
+
 # Should we keep the original file?
 keep_original = boolean(default=False)

[Christopher Allan Webber]

It sounds like a good idea. Another way we could do it would be to have a more generalized variable like media_types_disabled_uploads = foo... with the appropriate list.