Add support for Gravatars in Confluence both for inclusion and export

To the extent we ship Andreas' JIRA avatars inside of JIRA, we'll need a way for users to have consistent avatars across all Atlassian products.

A third party created the Confluence Avatar Server plugin specifically to "allow confluence avatars to be used in fisheye in an effort to maintain consistent identities across tools."

However, according to MattR:

One serious problem with that plugin: it loads all the users in the system at start-up, whenever a user is added or removed, and whenever a gravatar is requested. The email-hash-to-username mapping is also regenerated for every user at startup and whenever a user is added or removed. It may work fine in instances with a few hundred users, but will probably quickly grind your Confluence instance to a halt after that. The lack of concurrency protection on the mapping HashMap is also a grave concern, given it is potentially read and updated quite frequently.

I think it would be best if this functionality was implemented in the applications. It isn't particularly hard, but it needs to be done in the application to have acceptable performance, I think.

Therefore, we need a way for Confluence to at least publish avatars.