Fix target="blank" occurrences for GlAvatarLink

The following discussion from !143368 (merged) should be addressed:

• @justin_ho started a discussion:

  Question: Shouldn't this be target="_blank" (with the underscore)? I find it interesting that even the GitLab UI docs use blank and there are even a couple of occurrences in our code.

  Maybe we should create a separate issue to follow-up on that.

We should update both the GitLab UI docs and every instance in GitLab code.