Fix a11y tabbing through the blob header buttons

This was observed locally on Firefox.

The following discussion from !19932 (merged) should be addressed:

• @pslaughter started a discussion:

  issue (unrelated): I was surprised to see that tabbing doesn't work for some of these buttons. I think that's because some are <button> and others are <a>...

  20191112_blob_a11y_bug

  Let's create a follow up issue for this.