format_avatar function

[DiogoParrinha]

Original issue on 1.8 private repo: https://github.com/mybb/MyBB18/issues/19

To quote:

While we have a format_name function, I think we should also provide a format_avatar function (with the ability to set a default avatar).

I always find myself writing the code out and would be much easier in a function. Not only that, we could statically store the user avatar in the function so that when the avatars are called in the postbit (for example) scale_image isn't constantly called.

[DiogoParrinha]

Closing this as it's done.