tree: several nodes picked up and moved at once

m now adds the row under the cursor to a set of marks and steps on, so
mmm picks up three; M takes the whole folder, Esc or :unmark puts it all
back. Marks stay while the list walks, so they can be gathered from
several folders. p, :mv and Shift+digit move the whole set.

With a pointer, Ctrl- or Cmd-click marks a row and Shift-click every row
from the cursor to it. Rows drag onto a folder row, a crumb or a space
pill; a marked row carries every marked node with it.

POST /api/nodes/move takes { ids, parent_id } and moves them all or
none: every node is checked before any moves, sealing into a sealed
folder happens before the one transaction that moves them, and two of a
name get slugs of their own. The single move goes the same way.

src/routes/mod.rs

@@ -63,6 +63,7 @@ pub fn api_router(max_upload: usize) -> Router<AppState> {
         .route("/nodes/{id}", get(nodes::get))
         .route("/nodes/{id}", put(nodes::update))
         .route("/nodes/{id}", delete(nodes::remove))
+        .route("/nodes/move", post(nodes::move_many))
         .route("/nodes/{id}/move", post(nodes::move_node))
         .route("/import", post(import::import_root).layer(uploads))
         .route("/nodes/{id}/import", post(import::import).layer(uploads))