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>
Merge pull request #800 from goodbrews/imgur-api-v3

Update to Imgur API v3
moderators now have teeth, more at http://meta.discourse.org/t/moderator-permission-set/6307/5
allow pms to be targetted at groups
:s/moderator?/staff/g ... our naming was kind of crazy, renamed moderator? to staff
display admin/mod status on user page
staff rename forgotten
fix style
Trying to improve dashboard layout with the referral stats on the right
Merge remote-tracking branch 'upstream/master'