Attachments added before first manual Save are lost from the post

[markjaquith]

1. New Post
2. Add Media
3. Add content to post
4. Manually save post
5. Note that media is no longer attached to the post

OR

1. New Post
2. Add content to post
3. Allow autosave to fire, giving the post a "positive" ID
4. Add media
5. Manually Save Post
6. Note that media is no longer attached to the post

We need to have autosave:

1. reparent attachments uploaded on a temporary ID
2. dynamically replace the temporary ID with the new "positive" ID in the Add Media hrefs

[markjaquith]

(In [7532]) Autosave fixes for attachments from andy. fixes #6389

[markjaquith]

Found a case where it doesn't work.

1. New Post (don't provide content)
2. Add Media
3. Close the box, and wait about a minute
4. I think autosave tries to fire, but doesn't (no title/content)
5. The post_id in the Add Media links turns to zero.

[markjaquith]

(In [7533]) Don't replace temp_id for "add media" hrefs if we don't yet have a positive post_id