Update to Imgur API v3

Version 2 of Imgur's API is deprecated. Their documentation for v2 is no
longer online, and applications can only be registered under version 3.
Version 3 of their API has a slightly different endpoint but, more
importantly, uses a Client ID/Secret pair instead of an API Key.

This PR updates Discourse to use the new version of Imgur's API.

Signed-off-by: David Celis <me@davidcel.is>