3982 Allow translations of older admin posts

https://code.google.com/p/otwarchive/issues/detail?id=3982

1) The field is now labeled "Translation of" rather than "Is this a translation of another post?" because it's asking you to identify the post it's a translation of, not to provide a yes or no answer.

2) If JavaScript is enabled, when you being typing in the "Translation of" field, suggested admin posts will appear in the format Admin Post Title (Post #1). You can choose the post from the suggestions. The text field will go away because you can only select 1 post. (This is how choosing a tag synonym when wrangling works. It can be changed, though, so that the field will stay around.)

3) If JavaScript is not enabled, you must enter the numerical post ID in the "Translation of" field. The ID can be found in the original post's URL (http://test.ao3.org/admin_posts/#).

4) When editing a translated post, it will only show the post number, not the Admin Post Title (Post #1) format. This isn't ideal, but it's how the autocomplete currently works. (See adding a bookmark to a collection for another example of this behavior.)

5) If the post entered in the "Translation of" field does not exist, there should be an error message saying so.