Adds `image_alt` for index's `image` and `avatar_alt` for both the
default_author's `avatar` and any page's author `avatar`. For symmetry
with their "parent" picture, `index.image_alt` and
`default_author.avatar_alt` are defaulted and assumed to exist, while
`page.author.avatar_alt` is not assumed to exist, like its parent
`page.author.avatar_alt`.