Refreshing avatars

Hello,

This pull-request changes the way of storing avatars in the cache and fixes problem with non-refreshing avatars. Previous way was relying on userId/userEmail, so in case user changed his avatar on gravatar it had no effect in the app (it always used the cached version). Currently, avatar file is cached as Base64 string from the avatar url.
I'm wondering how to deal with old avatars. Storing all the avatars with some prefix (let's say: user_id__Base64) and deleting user_id__* before storing new one would work, but I like the idea of periodical service execution which will be maintaining cache state so it will never exceed rational limit - as the cache can contain also current avatars of the users, which are no longer active.
What's your opinion?

Best.