This way screen reader users will hear the name of the file before
hearing about the link to destroy it. We were already displaying it this
way visually by having the file name on the left and the destroy link on
the right.
Thanks to this change we can also simplify the code which dynamically
changed the layout.
Using `dom_id` means generating `new_document` as ID for new documents.
Since there might be more than one new document in the form, that means
duplicate IDs, which is invalid HTML.
Even though this issue doesn't affect image fields (because we don't
have many images on the same form), we're removing the ID there as well
for consistency.
These labels weren't associated with any fields, which is invalid HTML.
We're using a legend inside a fieldset instead, which is the natural way
to group form fields together.
We were using long, unique names because these methods used to be helper
methods. Helper methods should have unique names because otherwise one
method would overwrite the other.
Now that we're using components, we can omit the `image_` and
`document_` prefixes.
