files: a file belongs to a node, and a document holds its own

An upload while writing is now a file node of the document, through
POST /nodes/{id}/files, instead of a loose row of media that only the media
browser listed. It moves, seals, goes to the trash and comes back with the
document. Migration 0029 gives every embedded upload that had no node a
place under the document that embeds it (a living one before a trashed
one), keeping its media id, so no body changes; the sealed-folder triggers
now ask a document's folder for the files under it.

- nodes: `holds` and `place_for` say what may go where; a document holds
  files and nothing else, in moves, uploads and restores from the trash.
  A loaded document carries its files and whether its folder seals them.
- seal: a document moving into a sealed folder seals its files first.
- media: POST /media, GET /media and DELETE /media/{id} are gone, and with
  them MediaBrowser and :media.
- import: `plan/` beside `plan.md` comes back as the document's files, the
  shape export writes; an older export's media/ goes to the first document
  that embeds each file.
- sftp: a document's files are `<slug>/` beside its `.md`, listed once it
  has any, holding files only, moving with the `.md`.
- web: paste, drop and :upload while reading go to the document; the info
  sheet has a files section (Enter opens, i embeds, y copies how); the list
  shows +N on a document, za folds its files open, p drops marked files in.

docs/wiki/SFTP.md

@@ -33,6 +33,9 @@ directory.
   different first heading back renames it.
 - Files sit beside the documents under their own names. A later file with the
   same name is told apart by its id: `slides (42).pdf`.
+- A document's own files are in `<slug>/`, beside its `.md`. The directory is
+  listed once the document has a file, and can be copied into either way. It
+  holds files and nothing else, and moves when the `.md` is renamed.
 - Anything copied in that is not a `.md` becomes a file, and its text is
   indexed for search.
 - `rm` and `rmdir` put things in the trash, as a delete in the browser does.