resource/gitlab_topic: Support avatars

This change set introduces avatar support for topics.

To reliably detect changes in the avatar the avatar is hashed into the internal _avatar_hash attribute and a custom diff function is used to detect changes in the avatar if the filename stays the same.

I thought about comparing against the image returned from avatar_url, however I doubt that this would be reliable enough because the image might be changed slightly - e.g. cropped or compressed - especially when gravatar is used.

@armsnyder wdyt about this solution?