use vignette for generating avatar urls

@drsnyder 
Use `VignetteRequest::fromConfigMap` to generate vignette urls for user avatars. We can't use `fromFile` because avatars aren't stored as file objects, they're strings built from the user's id.
This awesome code was brought to you by: https://github.com/Wikia/app/blob/dev/extensions/wikia/UserProfilePageV3/Masthead.class.php#L215

Looks like there are a few different possibilities for a user's avatar: a valid relative path (`a/ab/<userid>.png`); a complete url (`http://domain.com/myimage.jpg`); a string without any path indicators (`/`), which indicates one of the pre-selected avatars a user can choose from (which comes from [here](https://github.com/Wikia/app/blob/dev/extensions/wikia/UserProfilePageV3/Masthead.class.php#L124)); or unset, which indicates the default avatar is used.